A lot of things. If we understand the variations we can better adjust medical treatments and educational methods to address the biological differences.
Call me a cynic, but I doubt that is why people do this kind of research.
And think about adjusting "educational methods." How does that happen practically speaking? Does is it justify giving preferential treatment to people who might be deemed inferior? Do you put people in a special ed class based on race (or mixed race)?
And what about the blood quantum? Someone has to be responsible for that. You cannot just go by how someone looks. Who is going to do, or submit to, genetic testing to determine their precise ethnic background? That's some straight up Gattaca stuff right there.