I'll tell you why I did. High settings with my old computer gave me too low of FPS on Crysis. If I dropped the resolution to 1600x900, it it looked horrible, as some pixels get doubled up, while others were single. Essentially, if you aren't running native resolution, it looks like crap.
It's like my HD TV stations. TNT and TBS does this a lot. If a show is not HD, they stretch it acrossed the screen and it looks horrible. It looks a ton better when they just give you black sides.
So I set the monitor settings so that it doesn't stretch the image if I use smaller resolutions than the screen is designed for. The monitor would just black out the edges if I set the resolution down, so it only happened on Crysis or any game I desired to lower the resolution on.
I do not have a need to do this now, as I have an i7 920 with a 5870, but I have done this in the past.