>Why climb the large mountain to store your grain there just to have haul it back down later?

Yes and after going on a trip to Machu Picchu a few years ago, the locals don't seem to feel gravity quite the same way most of us do these days. There was a gal on our 4 day hike that got hit pretty hard with altitude sickness a day in. A local porter about her size carried her on his shoulders for the rest of the trip, in flip flops, and the only reason he stayed back with our slow asses was so she could talk to her husband along the way.

It's the most visceral experience I've had in the levels upon levels of human capability. Really wild to see in person.

Also Peru is phenomenal.

Could also be a form of refrigeration if crops were grown in the valley but benefited by cooler temp storage at higher altitudes

Likewise it could have been snow/ice farming to have it available into the summer.

Not sure what the weather was like here that long ago but it’s another angle to explore.