In my first job - i think it was in 1997, I had my own small room with an L-shaped desk with a rounded corner. That gave a few inches of space for resting my arms - both when typing on a quite reasonable Pentium laptop, and especially when using the mouse.
Since then, the desks and the chairs has become shittier and shittier. Except perhaps when a was a consultant for an HR-department.
The U-shaped desk was probably the best ergonomically designed workplace I've had. Maybe a wheat-filled pad along the desk would have made it better.
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).