Because if people by and large hoard it it has a different impact than if they go spread it around in the local community and keep money flowing throughout the system. It's also good to know if they spend it on luxuries, entertainment, or basic necessities. Do they invest it and does it help them build a better future for their children, or does it just help them pay the bills immediately but doesn't necessarily lift them out of poverty, etc. Otherwise we can't tell if it helps solve the problems people are trying to solve with it (whatever those may be, I am not one of these researchers and am not qualified to speak on the economic specifics, I can just talk to general methodology and what not).
> People will spend it on whatever makes them happiest--that's the whole point.
Just specifically I want to point out the flaw in this reasoning: for a lot of people a $500 night out with their friends might make them happiest, or a new video game system or whatever it is that they like to do, but they may spend it on groceries or to help pay the rent or to invest a little bit instead. Or maybe not. You can't just assume what people will do with it, or what the whole point is, you have to experiment and prove it. Maybe something entirely different that you haven't thought of happens. Again, that's just how science works.
TL;DR this is an important thing to find out. Then we repeat the study in a few communities and see if the results are similar, maybe try it at different wealth levels to compare, etc. this is how science works.
> It doesn't answer the questions that most people have, such as the question in the post you replied to:
There can be more than one question, and more than one answer. Economists are researching this too and have written a lot about it. Just because you have a question, doesn't mean the science has to answer it right away and every single experiment has to be focused on it. There are lots of questions that will have to be answered before this could be tried more widely, these are just two of them.
But why does that matter? That's their own business. People will spend it on whatever makes them happiest--that's the whole point.
It doesn't answer the questions that most people have, such as the question in the post you replied to:
"Namely, why wouldn't prices/cost of living/rent rise to eat all the stipend, and how will we pay for it at massive scale?"