That's so some simpleton doesn't come in and accidentally save over a file that's important. "Hiding" files works similarly. It worked significantly better back in the days of DOS when it took 56 commands to open a file :P

(Naturally, the inherent flaw is that read-only-ness doesn't prevent accidental DELETION... :/ but Microsoft for you there, providing the "Hey, it's 'Recycled'--it's not deleted until it's deleted!" Bin... whatever)