Sound: 1 little note added or taken away can drastically change the sound of the chord. This can change the whole MOOD of the chord as well. A bright note added is a very different sound than a bright note taken away. Just think of it as using accents to your overall sound.
Convenience: Like Ben mentioned, it depends where you are on the fretboard. Sometimes it's hard to make a certain jump or change, so plan ahead and think of how you are setting yourself up for your next chord/note.
Actually, I'll also ad "comfortableness". Everyone's hands/fingers are different and how someone plays a chord might be almost impossible for you. So just do what your hands can do COMFORTABLY. Don't overstretch or hurt your hand fingering a chord a certain way just because that's how someone else does it.
Over time, you'll just find your preferred way of fingering a certain chord and do that all the time without thinking.