To put it succinctly you need to rub two pieces of wood long enough to get fire.
Likewise you need to meditate continuously for some stretch of time to see results.
As Goenka says "Continuity of practice is the secret of success".
buddhism.stackexchange.com/users/295/Suminda Sirinath S. Dharmasena
Q.What is gained by learning Vipassana meditation in long retreats?
A.
You get to observe your mind and body.
By observing sensations arising in your mind and body and knowing that it is impermanent, you learn not to react to it as you did in the past.
So, as you practice the art of non reaction to your sensation, slowly then you become free from all sufferings.
They key here is "non-reaction", no war whatsoever with mind (craving and aversion).
Just acknowledging the sensation as they are and non reaction to it is my friend, the only way to liberation.
Non-reaction again
i stress which i believe is the heart of this technique and logically is the only way to liberation.
You learn this technique in a 10 day retreat.
Its up to you to practice the technique for rest of your life and later after you get well versed in this technique 24 hours a day.
Note: If you are going to vipassana retreat as taught by Mr. Goenka, it is tough retreat and you have to work hard but very "gently" to train your mind.
It will not be easy but will be very worthwhile if you understand the technique properly.
I just give you sketch, the retreat will give you whole picture.
Wheel of dhamma is spinning, since it was first launched into action by samma sam buddha.
buddhism.stackexchange.com/users/2133/Pratik
Background.
15 years ago I went to a 10 day Vipassana Retreat.
I lasted 4 days.
10 years ago I went again and finished the Retreat.
I went for a 3 day Retreat this last winter.
It doesn't seem to get easier.
I have learned something different each time.
I had been meditating 30 mins.
early in the day (anapana not Vipassana)
4 months ago I decided to meditate 40 mins.(Vipassana)
twice a day.
I am noticing subtle changes ....
changes that are giving me insight into myself.
I see thoughts before they become words.
I see my thoughts before becoming engulfed in them.
It has given me space to see before I talk or act.
I am still a victim of my thoughts at times ....
but experiencing these subtle changes and the results has made all my effort a pittance in comparison to what I am starting to realize.
Perhaps I am a slow learner .... and my token gesture for daily meditation was not sufficient to realize progress.
The difficulty in the 10 day is part of the the process ... for me.
It helped me realize that I am capable of overcoming my thoughts.
I always had been one for short term rewards.
Maybe until 4 months ago .... I wasn't ready for a more dedicated practice.
Now I am beginning to make a conscious effort in my daily life.
Body sensations and anapana when not doing sitting meditation are something I'm trying to use more in day to day life .....
I still forget much more often than not.
It's okay.
I have seen progress.
Part of the process of progress is being okay with where I am now.
buddhism.stackexchange.com/users/8121/gary demos
You gain an intense and unbroken continuety in meditative practice.
That can be a deep and profound way to experience the dhamma through practical realization.
Especially in beginning of ones practice it is useful to get a point of reference .
Buddhism is big and wide and holds a tremendous amount of teachings.
One can read book after book but without any reference-point to reality, i.e. experiental knowledge gained through insight meditation, one can easily fall into the "believe or not-believe" category.
By doing an intense retreat with a teacher one will achieve insights for oneself that will serve to make one develop the first path factor of the noble eightfold path which is "Samma Ditthi, Right View" .
That is like our steering-wheel on the path.
If one does not achieve any insights on the retreat one will at least get a good feeling of what buddhism and meditation really is.
One will also have a teacher to interact with and thats a great thing since in beginning of practice there will be a lot of questions.
Other than that one will learn a lot about oneself in such an intense retreat.
One will learn about ones own limitations, sitting postures, habits, aversions, likes, wantings etc.
When one spends so much time with oneself one is bound to experience the content of ones mind.
That can be a profound experience, even just to get a glimpse of what really happens inside ones own mind.
One begins to see that phenomena are causally connected, that phenomena can cause new phenomena to arise and that objects will not spawn if the supporting conditions are not there or if they fall away.
In an intense retreat like the 10-day retreat one could also get to see the impermanent nature of phenomena on a deeper level than the day to day level.
So there are definitely a lot of advantages of these retreats.
There are also disadvantages as with everything in life.
Maybe one has bodily limitations that limit one from sitting for long hours in the same position.
Maybe one is stressed out and cannot focuz the mind.
The point is that even if there are "bad" things that will affect one on the retreat one will get to know these things and be able to work with them.
That is powerful because how often do lay-people have the oppertunity to really work deep on themselves in everyday life?
buddhism.stackexchange.com/users/2424/Lanka
I've done 6 days of the 10 day Goenka course after which I left due to irreconcilable philosophical disagreements both with Goenka and with the teacher.
Nevertheless, I found the course to be a powerful way to improve my meditation.
After meditating for 12 hours a day with no distractions, I found it very easy to meditate for an hour a day with the usual daily distractions.
10 days of meditation is what led Siddartha to enlightenment.
buddhism.stackexchange.com/users/None/
My understanding of S.N. Goenka's method for learning Vipassana meditation is that the student is introduced to meditation in a 10 day retreat in which he or she sits in silent meditation for almost all his or her waking hours. This seems  a daunting way to introduce meditation to a newcomer. Also, for many people, it could be difficult to arrange for 10 days without family or work obligations. Thus, I ask: What does the student gain by through an introduction to meditation via a "marathon" retreat?
What is gained by learning Vipassana meditation in long retreats?
buddhism.stackexchange.com
<vipassana><meditation-insight><beginner>
buddhism.stackexchange.com/questions/1336
buddhism.stackexchange.com/users/69/GreenMatt
You need to meditate continuously for some stretch of time to see results.  You gain an intense and unbroken continuety in meditative practice. Especially in beginning of ones practice it is useful to get a  point of reference. As you practice the art of non reaction to your sensation, slowly then you become free from all sufferings. Other than that one will learn a lot about oneself in such an intense retreat. If one does not achieve any insights on the retreat one will at least get a good feeling of what buddhism and meditation really is. A long period of time spent practicing is likely to enable the participant to progress and achieve results. participants will have the opportunity to explore their own minds and learn about their own limitations. They will also have the opportunity to develop non-reaction to sensations. For those new to meditation, guidance and interaction at the start of the journey will help to answer the many questions and lead down the correct path. Participants will find that phenomena are interconnected and the 10-day format allows them to meditate without the usual day-to-day distractions. This enables people to meditate better when they return to their normal lives.
A long period of time spent practicing is likely to enable the participant to progress and achieve results. Intense meditation  leads participants to explore their own minds and learn about their own limitations. They will have the opportunity to develop non-reaction to sensations. Guidance and interaction at the start of the participant's journey will help to answer the many questions and lead down the correct path. Participants will find that phenomena are interconnected and the 10-day format allows them to meditate without the usual day-to-day distractions. This enables people to meditate better when they return to their normal lives.