Check out the video. Those are the metates, or grinding mortars, that give Indian Bread Rocks its name. They used them to grind acorns, mesquite, prickly pear seeds, etc. But yes, they collect water, too.
At City of Rocks, there are skinny channels to collect runoff from some rocks. I think they'd put vessels where the channels drained.
Awesome video, thanks for sharing that! The hole he found water in is exactly like what I saw. I saw a big wash on my way down the mountain. I might go see where it leads.