Cats need to feel safe and secure and a nice, cosy bed goes a long way towards making them feel protected.
A special cat bed, a basket or even a cardboard box lined with blankets or old towels makes an ideal bed, provided it is warm and easy to clean. However some cats expect and indeed insist on sharing your bed!
This is usually not a problem unless they take over! If this doesn't suit you, close the bedroom door and make sure they have a comfy bed of their own elsewhere.