The main reason is the same as with a PC...Why would someone like more an 1280 x 1024 resolution on a 19" LCD screen and not an 800 x 600 one?
Because more information fits on-screen. If someone is comfortable with the smaller font size (eg. has good eyes), a higher resolution provides a better experience for those that need this. Who's not used with such small fonts or don't like it or have eye strain, could find better the normal resolution. Is only a matter of taste. Personally I prefer 200-220 setting. Standard 240 looks a little bit lower for me, and everything seem huge...I prefer smaller icons/letters and more "desktop space"