One could imagine any number of reasons: air quality, sunshine, a calming effect from seeing nature or variety in peripheral vision, difference in terrain provi

One could imagine any number of reasons: air quality, sunshine, a calming effect from seeing nature or variety in peripheral vision, difference in terrain providing stimuli, strength benefits of carrying the apparatus (it sounds like he often carried stuff even before working outside) The benefits of the outdoors are pretty widely reported. And it doesn't sound like he went outside much of his own inclination.