There have been several threads about this (though the search function wasn't working a moment ago), and I think the consensus is that they just want to get a broad idea of who you are as a student. College is only an isolated 4 years, so they probably want to see general trends, which courses you took, what you were interested in, etc. It's probably more to study you as a person than actually look at your high school GPA. A few other schools want HS transcripts as well. Just one of the annoying things to get done! 🙄