To see how cool it is. Try it with a quicktime movie, while it is playing. The entire point is to show how cool it is. My theory is that they put it in there to demo the feature, and thought it was cool enough to leave in. Almost an easter egg, but since it was probably steve's idea, it probably wouldnt count as an easter egg.
Steve demoed it when 10.1 was released just to show of quartz
but it works with anything, shift cllick on a folder and it opens slow, shift click and close one it closes slow, and go to get info and click on some of the arrows (this is in jaguar BTW) and they go in slowmo
Maybe OS X had the shift key held down using software the whole time and they used some clever coding technique to stop applications other than the finder responding to it. Now they've got rid of the "Feature" it makes OS X seem a lot faster
Seriously though, that's a good idea, I mean for a start it gives you one more thing to show PC owners and then have them claim the PC did it first when windows does it in 5 years using "bloat Xtreme" or whatever they end up calling whatever cheap rip off of quartz they come up with.