It's commonly used by Adoptees to find their biological family. This can be important for a few reasons, including finally getting accurate family health history.

Why would you care who your great great great grandpa was?

I was able to find out that my great grandfather was adopted, and meet a whole new wing of my family. I didn't even do the test, my aunt did.

About 10 years ago they provided medical data from the samples. I used 23 And Me too confirm that a health problem I'd recently been diagnosed with was hereditary. At the time I remember being asked if my sample could be used to aid the type of research the OP talks about and I agreed to it.

A couple of years ago, I think 23 And Me was bought out by Virgin Healthcare, at that point I asked them to destroy all my data was worried about it being used to increase the cost of or preclude health insurance.