It's a 4GB disk image, the contents of which are copied to a new partition on your hard drive.
Why would they keep it around? They aren't deleting it just to make your life harder ...
Edit: Looked into it and I was going off bad info. The "Recovery" HD isn't really a partition (it is a disk image), and it isn't the entire installer DVD, just about 750MB worth.
Still, I wouldn't want a 4GB installer sitting around forever.
Why not, I never burned my Snow Leopard to an actual DVD. It sat as a DMG on an external hard drive for ages, until I finally made a bootable flash drive from it last week.