My ad-hoc list of reasons for using a minor dominant in a minor key.
Milder, more ambiguous feeling - it doesn’t shout ATTENTION ALL LISTENERS!
THIS IS A DOMINANT CHORD.
I REPEAT.
DOMINANT CHORD.
Make your music cooler and more suitable for young rebels/snobs/kids who cannot stand proper dominants, because it reminds them of the kind of uncool music old farts like their parents listen.
Different jazzing-up possibilities.
If the melody is compatible with it, try a minor 9 or even a IV/V (i.e. D6/E for Am)
If you want to make a harmonic variation or arrangement of a melody that was written for a major key, and use the relative minor key instead, the melody might clash with a major dominant’s third.
So use a minor as a dominant - it’s a dominant all right.
Bass player’s revenge / application of above: if you’re tired of a happy C - F - G - C song, play a third below and make it Am7 - Dm7 - Em7 - Am7.
music.stackexchange.com/users/51766/piiperi Reinstate Monica
Yes.
i v6 iv6 V where the second chord is the minor dominant in 1st inversion.
Harmonizing a descending bass in that way would be very normal.
The important point to realize is that the proper dominant V is used at the end of that example phrase to create a half cadence and in that place the raised leading tone is required to make a proper cadence.
If the music isn't forming a cadence then the treatment of the ^6 ^7 scale degrees become flexible in minor key music.
That sort of flips your question around.
Rather than a reason for using the minor dominant, the reason for using the raised leading tone is what matters.
You don't need reasons and rule for using any chord as the music unfolds.
But, when you get to a phrase ending and form a cadence, you must follow the formula.
At least in classical style.
On the other hand, if you aren't dealing with classical music, then such supposed rules and theories are simply being mis-applied.
music.stackexchange.com/users/23919/Michael Curtis
Though I'm probably just restating Tim's comment - getting the leading tone in isn't necessarily a good thing.
There's a proud tradition of music oriented around the leading tone - a tradition that came about ultimately because people thought it sounded good!
People still do of course, but they also want variety, and often want to get away from that insistent leading tone.
Sometimes this is done by avoiding playing thirds altogether, including when playing a chord on the dominant (by playing 'power chords'), or even by avoiding the dominant (a characteristic of 'grunge' music).
Almost any time you find a guideline in music, You'll find a bunch of people who prefer the sound you get when you don't follow that guideline....
music.stackexchange.com/users/18896/topo Reinstate Monica
Even in the most tonal harmonic minor pieces, the leading tone is often avoided unless the melody is specifically moving upward toward the tonic note.
That means there is a lot of opportunity for a minor v chord that involves the un-raised 7th scale degree.
Feel free to use the minor v wherever you want unless you want to create a strong cadence.
music.stackexchange.com/users/47856/Heather S.
In Common Practice Period theory, the minor dominant (AKA minor chord on scale step 5) is rather common.
It's used when harmony based on step 5 is wanted but without the cadential implications of the V chord.
Note that in a minor key, the V has a stronger cadential tendency as it contains the raised form of step 7 and this note has a strong melodic tendency to move to step 8 (at least in that style.)
There are two common uses; one is the one already mentioned: a descending bass i-v6-iv6-V or i-v6-iv6-v, etc.
Another is in sequences like the minor form of the "Rule of the Octave" (AKA the Pachelbel Canon harmony):
I-V6-vi-iii6-IV-I6-V6-I (and many variants).
In minor one gets i-v6-VI-III6-iv-i6 or (I6)
etc.
The other is in the cycle of fifths, i-iv-VII-III-VI-ii0-v-i -i-iv-VII-III-VI-ii0-V-i.
The first time through, the v chord does not have a strong cadential effect (more like a secondary dominant) thus contrasting with the V in the last time through to achieve strong cadential effect.
music.stackexchange.com/users/23838/ttw
I'm sure there are more insightful answers than "it sounds good" Not really.
You can have a functional V with the leading note included, or a less functional v with the b7 of the key.
Because it sounds good.
Popular music lives in a sort of modified Common Practice harmonic world, where even in a major key the b7 almost counts as diatonic, probably because the Blues is constantly in the background.
music.stackexchange.com/users/17003/Laurence Payne
I've learnt that in a minor key, the non-diatonic major dominant is basically always used instead of the diatonic minor dominant in order to get the leading tone in. Of course this is a trend but not a rule. What could be reasons for using the minor dominant?  The question was spurred by Amy McDonald's This is the Life, which goes i-VI-III-v in C#m. (I'm sure there are more insightful answers than "it sounds good".)
What are reasons to use the minor dominant in a minor progression, in particular i-VI-III-v?
music.stackexchange.com
<chord-progressions><functional-harmony>
music.stackexchange.com/questions/78110
music.stackexchange.com/users/47563/Anna
If you want to make a harmonic variation or arrangement of a melody that was written for a major key, and use the relative minor key instead, the melody might clash with a major dominant’s third.  The important point to realize is that the proper dominant  V  is used at the end of that example phrase to create a half cadence and in that place the raised leading tone is  required  to make a proper cadence. Even in the most tonal harmonic minor pieces, the leading tone is often avoided unless the melody is specifically moving upward toward the tonic note. That means there is a lot of opportunity for a minor v chord that involves the un-raised 7th scale degree. The minor v can be used wherever you want unless you want to create a strong cadence.   Feel free to use the minor v wherever you want unless you want a strong cadence, but avoid the un-raised 7th scale degree.
Harmony in the minor key can clash with a major dominant's third in a major key melody. A proper dominant  V is used at the end of that example phrase to create a half cadence. Feel free to use the minor v wherever you want unless you want a strong cadence, but avoid the un-raised 7th scale degree.