DPI buttons are for changing cursor sensitivity (speed) without going into software - usually between a few customisable presets.
So if you do graphics and want to switch from regular/high setting to a lower setting for greater control.
Also handy when watching videos, you can set it to super low so that knocking the desk doesn't make the mouse move slightly and bring up the control panel.
Or if you need to let one of those people that can't use a mouse without moving their hand and arm loads use it.