1st reason - It's polite to sell a Mac with an operating system already installed!
(and not a blank hard drive...)
If you are booted to the installer disk, and use that to wipe the hard drive, then it's a simple matter to just continue on with the install/restore. Do an Apple-Q when the install is complete and you see the first 'new user setup' screen. You can then shut down without running that setup, and the new owner has a brand-new fresh install of OS X (and none of YOUR stuff, eh?)
2nd reason (Aha! the real reason!) - you can't erase the volume that has your running operating system while you are booted to that system. You can do that while booted to a different disk, and the install/restore disk gives you the convenience of installing a fresh system (as noted above)