Because they get to play it early. That's it. I played BG3 early and still had a lot of fun replaying Act 1 when it came out.

The studio gets a number of things, as well. While direct feedback is small, that is still valuable as they could never test that many hardware and software variants. They also get automated data from the software phoning home on crashes if that's enabled. And they get an influx of cash in the last few months of development as their sales spike gets a bit flattened. It's a winning strategy if you don't have the funds for a huge marketing blitz to drive initial sales.