From MacTutor fave www.cultofmac.com: See, the Mountain Lion installer is set to self-destruct once it’s used. When your Mac reboots running Mountain Lion, the installer will be gone and you’ll have to re-download it if you want to use it again. Another reason to create a bootable copy of Mountain Lion is if you want to do a clean install. Apple’s installer will keep your files and personal setup in tact while installing Mountain Lion, but a bootable disk will allow you to erase your Mac and start completely anew.