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.