Do rats need houses inside their cages?

No, but they love them. They like to jam themselves into small boxes. If you can get 7x7in. plastic boxes with holes, they can be washed. e.g. baby-wipe boxes, plastic corn poppers, newspaper rural-delivery tubes, 1/2 gal. milk cartons, old Playskool toys w/o bottoms. Plastic/PVC sewer pipe connectors come in many shapes and rats love to climb around them and sleep in them, just remember to get a size that will fit all the rats in the cage. It's best not to give a new rat a box to hide in, as they might become territorial and attempt to nip when someone tries to reach into their 'den'. Wait until the rat already knows you and wants to be with you.
Note: Plastic will be chewed by most rats and if this causes concern about possible ingestion, then there are wooded boxes available. Parakeet nesting boxes work pretty well if you are careful to not catch their tails when opening or shutting the lid. Hole may be cut larger for male rats or pregnant females, if they don't chew it to fit themselves.