The landing strip theory is pretty much dead from a purely engineering standpoint and I think most serious researchers have quietly moved on from it. The lines couldn't support any aircraft we know about, the surface is just compressed gravel and would collapse under any real weight, and Erich von Daniken basically poisoned the well for everyone by packaging it in such sensationalist terms.
That said I don't think dismissing the "made for aerial observation" angle entirely is unreasonable. Not aircraft necessarily, but something. The sheer scale of the geoglyphs makes no sense if they were purely ceremonial ground-level markers. You wouldn't design something that intricate if you never expected it to be seen from above.
The water/ritual irrigation theories are interesting but they feel like they're bending over backwards to avoid the obvious question which is why build something you can only properly perceive from altitude. Would genuinely like to hear what others think on this, particularly anyone whos looked at the newer LiDAR survey data that came out a couple years back.