Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
The utility is that absolutely everything from the prior install is still extant on your machine. That way, if there's anything in the old System directories squirreled away that might need to be copied over for some reason, it's still there.

It was never intended to be re-used. If you need to maintain the Tiger boot, then you should do a clean install of Leopard on a separate partition/drive.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.