What is the purpose of the Shift+F3 keyboard shortcut?
The shift modifier has been around for most animated sequences since at least Panther (10.3).  I don't know that there was ever an official reason given but it's widely assumed that it was built in at the request of Steve Jobs in order for him to highlight Expose' as he announced it to the world at the 2003 World Wide Developers Conference. You can see this announcement here, with the slow motion effect being used for the first time around the 2:00 min mark.

Mac OS X Panther - Expose demo

Given the crowd's reaction I think it's clear why Apple didn't remove it from the final version.  It's always been one of those little easter eggs that Mac users use to impress their PC using friends.

So there you go, no practical purpose, just a bit of fun for Apple.  You can also use shift to slow down most functions that have an animation.  Minimizing windows, opening Mission Control, etc.