keyboard, wireless: Because the amount of energy it uses from the batteries is minimal, and as such those batteries last a long time. When the batteries are low/out - they keys don't work, and it's obvious that I need to change them.
mouse, wired. Because I've found I have to change these batteries fairly often, and it's not just that the mouse suddenly stops working, I get a gradual reduced performance that I don't usually notice until it's really annoying.