I installed a game through the original game disc but I still need the disc for some reason. Why?
Installing the game on to the 360's hard disk, simply put, does nothing more than reduce load times, as the image on the hard disk can be accessed much quicker than the image on a DVD drive, thus increasing the performance of games that need to frequently load content from the media.
You still need to provide the game disk however, for copy protection. Installing the game can also help when your disc is scratched.