You'd be surprised by how much seemingly obvious folk wisdom turns out to be bullshit when scrutiny is applied. It's good to test the basics, as it can clear up faulty assumptions later. Anti-poor rhetoric is a staple of right-wing propaganda, so it's nice to have research on our side.

In addition to what others have said, this paper serves to directly address a very common talking point.

The thing with science is that you can't just accept things because they seem obvious. The scientific method exists for a reason. Sometimes things that look obvious turn out to be false, and sometimes proving an obvious thing to be true is a necessary first step to have a solid foundation from which to build other more nuanced hypotheses. Either way, the point is that studies aren't all about finding some new and surprising conclusion. Sometimes they're about taking something you were pretty sure of already, and making it into actual science.

Yes, in fact it's the most obvious things which need scientific study most desperately, because what's "obvious" today may tomorrow be "cigarettes are good for your health". If something is obvious (and isn't a value or opinion), that's a red flag that you should double-check because it might be an overlooked assumption rather than an objective truth. That being said it's not exactly "sexy" science since the result tends to be "our hypothesis was correct". The Ig Nobel Prize has some good example of this, dumb "obvious" or "pointless" research that on further reflection makes you think.

It's more like we have a hundred years of studies that make this point, but it's always good to test for different causes of inequality to inform policy better.