Why? Can't think of any deeper reason than its just a bit of fun.
The state of the art is not quite Toy Story, but its certainly come a long way. I remember playing with (now defunct?) JavaHead a few years back: impressive demo, but it was hard work to get good results with your own images. You needed to be a techie/animator/audio engineer.
I've just been playing with PQ Talking Photo, and it's great to see not only a pretty good, synchronised animation but also a great, anyone-can-do-it UI. My little test is below, hopefully earning me a promo copy by blogging about it. Shameless!
Edit: pqdvd is defunct, the embed above no longer works.