I used to have a Surface and the one thing I would use the touchscreen for while sitting at a desk was to scroll with a thumb on the lower corner of the screen (with my arm resting on the desk).

It's nice to be able to change position and have your arm extended along the side of the computer sometimes instead of bunched up next to you to scroll on the trackpad.

Beyond that, yeah if I'm at a desk with a keyboard and trackpad, I'd rather use the trackpad than hold my arms up in the air on the touchscreen.