I don't know much about the Saguaro community but it's probably so that they can protect themselves or perhaps because of the sort of small animals that are there? So that they can feed themselves.
“Such nonsense!" declared Dr Greysteel. "Whoever heard of cats doing anything useful!"
"Except for staring at one in a supercilious manner," said Strange. "That has a sort of moral usefulness, I suppose, in making one feel uncomfortable and encouraging sober reflection upon one's imperfections.” — Susanna Clarke, Jonathan Strange & Mr Norrell
