Some people are useful. Some people are really not. These people are as much use as a stray penis in a sacred nunnery where all potential partakers of the penis have actually given the solemn vow of chastity. Therefore the penis is really of no use. Hence the simile to suggest that someone is really useless.
That guyCarl is as much use as a cock in a nunnery. He is absolutely useless
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"