Well, after a lot of limps or a 3bb raise with a lot of callers you may want to push your aces. If everyone folds you have a nice pot anyways. A small re-raise may get more people in and you don't want 4 other people in the pot with aces. You aces may be the best hand, but you won't have over 50% most of the time. With a all in, you hope 1 or maybe 2 call and have nice odds
to win the pot. If everyone folds you will also have a nice pot. Deep in the tournament with a nice bb, sb and ante in the pot you may also want to push with a small stack. In any other situation i would just go for a re-raise and not go all in.