They don’t create supply in any way, the only ones who do that are builders. But sure they maintain houses. Although just the bare minimum, they will never fix it nicely - just enough to rent it out.
For a house to be available for me to rent, both things need to happen.
Someone had to build it, obviously. But just as necessary, someone needs to offer it up for a rental.
Building often happens because a developer believes someone will buy the homes they build. A lot of the time, they are right; some of the time they are wrong. Some developers will get buyers lined up first, to minimize the risks of being wrong; many do not. In the latter case, there's no inherent connection between the building and subsequent buyers.
Depends a lot on the landlord. Many will fix it up nicely because they can charge a higher rent. Much of my work is repairing rental properties and I've seen all types of landlords. I try not to work for the cheap ones if I can help it because I don't want my name associated with the crap they want me to do.
I spent over a decade living in various rentals after I moved to a new state. I didn't have the money to buy when I first moved, and even if I had, I didn't know the area well enough to know whether I would want to buy where I first lived. And having the ability to just pick up and move meant I had a lot of flexibility for chasing job opportunities. Don't get me wrong, there's plenty to love about the home I own now, but it absolutely ties me down and anchors me in ways that renting never did. I for one am glad to have had people willing to rent property to me.
Like I said in another reply, I don’t consider landlords to be inherently bad. But there are a lot who will try to take advantage of you if you let them. You have to be lucky to get a good one - I only had a couple out of the dozen or so and I wish them the best.
A renter is someone looking to rent. If someone buys a home then rents it out they just +1 the supply of rental units.
A high proportion of real estate sales are owner churn, not the purchase of brand new never used before properties.
I just can't bring myself to agree with the hard-line socialists who think landlording is fundamentally a bad thing. There are a lot of problems with it, but it does have a legitimate place in the world.