There are two hidden bonuses gained by a pilot walkaround:
Firstly it brings you up to speed with the current wx, especially freezing mist etc. Also the state of the ramp.This may not be apparent from your nice airconditioned seat with windows at the front.
Secondly it's an opportunity to say hi to the ground staff such as tug drivers, bagage handlers, refuellers, caterers etc. It's surprising how often they will pass a bit of information on via the headset man when they think you are a half way decent chap/chapess.