Sinks are used to clean dirty Food Trays. The used trays are piled up after the meal and get cleaned by a Cook if there is time. Sinks are required in every Kitchen and need a Water Supply. If attached to a Hot Water Pipe, sinks will work more effectively.
If there are not enough sinks or cooks to clean all of the trays, then there won't be enough for the Prisoners when meal time arrives.
The description of the sink in the game says "All employees are required to wash their hands". However, sinks are only used by cooks and prisoners to clean trays in the kitchen, and nobody will ever be required to wash their hands. Don't bother placing a sink in a Staff Room.
A cook cleaning up dirty food trays.