Say you're rendering out a nice big avi and it takes ohhh, about a day and a half, and you're just getting to the end, and maya decides to crash. All you've got now is a corrupted unfinished avi.

So... render an image sequence and you lose at most a single frame.

Also, say you want to re-render a short segment. Just re-render the frames you want instead of the whole avi.