If you have the drive space, you may want to consider installing Halo: Reach to your Xbox 360, as loading some game types can go nearly two to three times as quickly.
The time savings are a matter of seconds, not minutes. But this comparison, by Examiner.com, shows the opening mission loading in 8.1 versus 21.4 seconds, installed vs. disc; Firefight loading in 21.6 versus 45.1 seconds - the biggest time savings; and a custom game loading in 9.7 seconds, as opposed to 28.7 seconds.
Another reason? Maybe it helps you avoid those nasty disc-read errors. I dunno.
The installation takes 6.6 GB, consistent with most game installs. The test was conducted on an Xbox 360 Elite. It did not specify if it was the new Slim configuration.
Halo: Reach Load Comparison, Installed vs Not Installed [Examiner.com]