Water for the dead

Visited a cemetery here in southeastern New Mexico and noticed bottles of water on two different graves. The tomb stones were almost next to each other. The bottles were soda bottles with the labels still on them, Pepsi I think. Anyway they were on the base of each tombstone.
My mother pointed to one of them having fallen over and told me to put it upright. I did. Most of the water was still in the bottle.
Very curious since the water was clearly intended for the dead people in those graves. Had never seen that before.