More like to keep the thieves out. Expecting elderly stoners to do it might be a stretch too far. It is unlikely that they would be able to see the thieves, much less do anything about them. Not much they could do curled up in the shade (if they could find any), doobie partially done and cold (the last one, if not the first), eyes glazed and the color of tomatoes (except they are closed so who knows?). There is couch lock, and then there is anywhere lock.
Electric fence, always alert, always on duty.
And, you are right, it would avoid Auntie spending time rounding the stoners up after her neighbours complained about the grey hairs in their gardens again.