What does downloading a game to my Xbox 360 HardDrive do?

So i have a slightly damaged MW2 disc (there is a perfect circle scratch on the disc) that works fine until I try to play a game. It loads the map and I choose my cl then it works for about 5 seconds then freezes and says the disc is unreadable. If i downloaded MW2 to my harddrive (like not bought it online, but just downloaded it from the disc) would that help? I know it makes the run time faster and reduces the amount that the disc spins. What do you think?