Here is what I learned since I posted the page:
"It is a ritual derived of the cult to Difunta Correa, an Argentinean mother that died of thirst in the desert near San Juan (Arg.) at beginning of XIX century and was found by a gaucho with her little baby still alive sucking her breast. Despite the Catholic church dislikes this cult, it is revered by truck drivers along Argentina and Chile, and now extended to all drivers. As homage to this poor mother fresh water bottles are put in shrines
More info Google "Difunta Correa."