Radiation is a big one. Earths magnetic field shields a lot on the ISS.

It's also about finding the things we don't know yet.

As for benefits of sending people. All of this has to be planned and constructed. People do that do get paid. The Apollo program for example returned $7 for each dollar invested: https://space.nss.org/settlement/nasa/spaceresvol4/newspace3.html

> Compared with other forms of investment, the return is outstanding: A payback of $7 or 8 for every $1 invested over a period of a decade or so has been calculated for the Apollo Program, which at its peak accounted for a mere 4 percent of the Federal budget. It has been further estimated that, because of the potential for technology transfer and spinoff industries, every $1 spent on basic research in space today will generate $40 worth of economic growth on Earth.

Some new tech will have to be developed which will be useful here on earth. So forth and so on.

The "problem" is that it doesn't benefit singular investors but society at large. So private companies won't finance programs at such a scale and you need to do some thinking to see how those benefits flow back into your pocket.