On va présumer que c'est du Jazz ?
La première justification harmonique, c'est bien sûr comment ça sonne. La théorie jazz ne fait en sommes que souligner son utilisation (puisque qu'il y a pas de "règle" en jazz comme dans la musique classique).
Citation:
1. Make the VI Chord Dominant
In normal diatonic harmony you’ll usually find that the vi chord is usually minor. In jazz though, we try to go for more of a chromatic sound.
So, we usually change this chord to a dominant seventh chord. Not only that, but we also usually add altered tensions to this chord.
If you need more information on how to play altered tensions you should also check out this dominant chord extensions lesson.
For example, instead of an Em7 there we will play an E7(alt) instead. This makes the minor II chord that is played afterward sound very strong.
http://www.freejazzlessons.com(...)ords/