"Without landlords/rental properties, more people would be homeless." True or false?

Please explain the rationale (and ideally data/evidence) behind your answer…I’m not looking merely for opinions, I’m wanting to understand how this would actually play out.

Someone I was talking to said that landlords and rental properties offer a more affordable housing option for people, and offer better incentives for builders to build, and without that, more people would be homeless.

It didn’t seem quite right to me, but I’m no expert on how economics play out. Is that what would happen? Or would the market adjust in some other way?

I guess what I’m asking, really, is: To what extent are landlords a necessary component to how the housing market works?