Also, I don't think it was specifically mentioned here yet - a good grinder will deliver more consistently sized pieces - fewer very fine and large bits, more of the output is within a narrower range of size. (I'm not sure exactly equal sizing, even if it could be achieved, would be ideal just because the packing would be inefficient and more gappy than with some variation.) One of the raps against the bad old blade grinders is the huge variation in grind size - burr grinders avoid that to some extent - better ones more. So, a well designed grinder can be adjusted with some confidence to tune up your coffee according to roast, bean, brew method, whathaveyou.
A better grinder may also have larger burrs that spin less quickly - thus the grounds are not heated, or not as much, which is a good thing.