Like a person playing the piano.

If your arms are resting, then your fingers and wrists are doing the maximum amount of reaching as you type. If you use a wrist rest you are encouraging your fingers/wrist to reach up (bend in your wrist) instead of neutral or reaching down (more natural position).

A more concrete way of putting it is if you are putting so much weight on your wrists that the edge of the MacBook is making you uncomfortable, you're probably doing it wrong.