What is the purpose of a bin?

I've checked the wiki and can't find any information on what bins actually do in Prison Architect (though it is possible they don't have function yet). The in game tool tip is similarly vague:

Cleanliness is important when it comes to contraband

I've tried placing them near to metal detectors but contraband never seems to be put in them, just dumped on the floor until it despawns.

Workmen will actually put trash into the bins. Contraband, is thrown on the floor by the guards, then picked up and placed into the bin by the workers. It also collects any other garbage from nearby sources, such as any food dropped on the floor by your kitchen staff. The cooks will pick up the "Ingredients" and put them in a nearby bin, instead of bagging them.

They really don't have much practical function, however, they do cause there to be less trash bags laying around your prison.