It improves visual quality and lessens the chance of your computer getting 'tearing' during 3d apps. it also enhances your resolution changes (1280x1024@60hz is very different than 1280x1024@100hz). The refresh rate cooperates directly with your video card, so as I see it, it would be best to keep it as high as safely possible at all times.
60hz will make apps with v-sync lock at 60fps (like HL1 defaults to, for example). 60fps is great and all but when you could have as many as 100/120 its very foolish to leave it that low, if you ask me--especially since it burns your eyes, hurts your eyesight, causes migraines, and affects performance.