There are tendons going from your fingers all the way to your elbows. These tendons must traverse the narrowing that is your wrist. Tendons are surrounded by a sheath that lets them move freely in one dimension as they extend or contract. There's two sets of these tendons: one below and one above, for bending and extending your fingers, respectively. On the underside of your wrist the tendons for bending your fingers all get close together. Now when you rest your wrists as you type you put pressure on all those tendons and their sheaths, and this causes trouble by increasing friction -- eventually scarring.

Don't do it.