This is an interesting way of seeing rental housing rents (national scale). And there’s a lot that you can glean from a mapping like this. But it’s also interesting in that what you are seeing here is a visualization of some 11 million Craigslist rental housing listings (taken from this study). The authors refer to it as a “nontraditional source of volunteered geographic information”, and they argue that it’s probably more granular and real-time than what is typically available when it comes to rental housing. That sounds right to me.