The idea is niche marketing. If you have just ONE website for buyers,sellers,investors,renters,and on and on it is unfocused.
So you would create a website just for renters like
www.newyorkrenters.com etc.
You would make the website THE RESOURCE site for renters to go to for finding properties. You would offer to put up properties for free on your website to draw renters. You could also convert some of the renters to lease purchase or also options down the road for sales depending on the rental type.
Then drive a bunch of traffic to that site with one way links,blogs,newsfeeds,etc.
If you have one website with so much stuff on it for everyone it's hard to find what they need quickly and effeciently. By having rich content of only that subject it will get ranked better as well for those keyterms as being more of a match.