Mice with variable DPI are useful when playing FPS games because different guns may be easier to use with a less/more sensitive setting. For example, sniping, you will need slow, careful movements so a lower sensitivity will slow your cursor down while normal run-and-gunning will requires fast movements that are better suited for a high sensitivity mouse. Also, with high sensitivity comes better accuracy when moving fast because your mouse "updates" faster. Other than FPS games, I can't think of much of a reason to have that feature.