because the charge in the air is slower.
if the opponent already has some damage, you can get a splat from a partial charge in the air... which honestly i do a lot, both because it gets me a cleaner shot of them and because it's fun as hell. it doesn't really give much of a benefit, though, unless you play the squiffer or maybe the bamboozler and any bit of range makes a difference.
actually, against other chargers it can get you killed pretty quickly, since jumping/airborne enemies are often the easiest to hit (but i still do it sometimes to switch things up and because it feels snazzy).
one thing that works well against non-damaged opponents is starting a charge on the ledge, then leaping off near the end of the charge. it's also risky (re: airborne target), but it can be useful to get a better angle on some enemies who are lurking in bad areas (and it's much safer to have a charge ready before you land than to land first and then charge). in fact, even against a slightly damaged opponent i'll usually start my charge RIGHT before jumping.
for example, i get a lot of aerial shots against climbing opponents. if i know someone is trying to climb up to my ledge, i walk backwards towards the ledge while charging and hop off backwards, releasing the fully charged shot in the air.
you have to be careful to jump BEFORE you finish charging, though -- walking speed with a FULL CHARGE is much slower and you won't have much momentum your jump.
