> People need houses to live in

I'm sure very few owners of multiples houses let them empty and not rent them out so that people can live in them.

Houses are expensive, even outside their speculative value, in terms of raw materials, workmanship etc. So expensive that few people can afford to buy houses with cash.

If someone is paying that cash to build a house, then offering it for an affordable rent so that other people can live in them, they are providing a valuable service.