I frequently see white PVC pipes sticking out of the ground near trees. Normally it's one, like by the tree on my property, but sometimes there's even two:

A closer look at the pipe reveals it's an NDS sewer & drain pipe.

What are these? Obviously not sewer, and they don't look like they drain anything. They look a little like DIY solutions I've seen before to get water deeper underneath the tree to train its roots to go down to look for water instead of up & destroying the landscape, but not quite since it looks difficult to pry off the cap. Is that what these are? Why would they need two for this?