Beds are used by prisoners to satisfy their "Sleep" need. Every Cell and Family Cell must contain a bed, and it is optional to have them in other rooms like the Holding Cell. They can also be used in dormitories as cheaper alternatives to the Bunk Beds. Beds are used by prisoners during the sleep time in the regime. Sleeping in a bed also has the added benefit of satisfying a prisoner's "Comfort" need.
Update The Bucket added following beds to the game:
It can be helpful to put a few in a Holding Cell so that Prisoners can satisfy their "Sleep" need. This will also increase your prison's official capacity and thus its valuation in the according Report.
Though there is a Superior Bed that your prisoners can make in the workshop, it cannot be used to replace the normal beds in their cells.
The Bunk Bed can not be used as a replacement for single beds in ordinary cells. You can however, designate the room as a dormitory and use bunk beds there. The other way round it is possible to use normal beds instead of bunk beds in dormitories.