Delta-v
As an example of how delta-v is used, we can imagine a goal: flying a spacecraft into low Earth orbit. (In simple words, getting a rocket to spin around the Earth). We would first calculate how much delta-v will be required for that journey. Then, with the factors in the above list, we can calculate how much delta-v our spacecraft would need to go on that journey. Like if a person were to drive his car from point A to point B, he'd need to know how much fuel he needs in his car first. But in rocket science, using delta-v is much more relevant, as we have to completely design our spacecraft, including all factors mentioned above.