I'd assume his reason would be that at a lower resolution, the load on the vid card/system wouldn't be as great.
If you edit the _Civ4Config.ini file in ..\Firaxis Games\Sid Meier's Civilization 4 or wherever, you can change to a custom resolution.
Change the 0s to other numbers, though I must say, I haven't actually tried this.
ScreenHeight = 0
; Custom Screen Resolution, ex: 1200x1200 - Normal resolutions can be set in-game and will be used when both values are set to 0
ScreenWidth = 0