its been like that for a long time. ever since you could minimize windows into the dock the shift key has made it go in slow-motion. why? no idea, but its fun.
- I know what it is, I just don't understand the point of it.
I suppose PlaceofDis' comment sums op the matter quite well: there is no point, but it's fun...
If I recall correctly, it was put in so that Steve Jobs could demo the behavior of the dock animations when OS X was debuted. It's probably still there just to be cool.