That’s not a dumb question at all.

There are MDs who do substantive research in medicine and surgery. However, basic science questions and a lot of the applied research are done by MD/PhDs. The dual degrees are a signal that the person holding them is not giving MMR vaccinations to kids or fixing broken arms, but who are committed to doing active research. It’s not an absolute requirement - I’ve worked with brilliant MDs at the VA and UC facilities who are gifted researchers. I’m painting with a very broad brush there.