Frankly, there still doesn't seem to be a compelling reason for Star to be on Earth. She's fighting the same monsters she would have been fighting on Mewni, and no one on Earth has posed the kind of threat to her that would educate her in the ways of the world. No one except Brittney.
Brittney potentially could have offered the kind of challenges to Star that can't be solved by magic, which was presumably why she was sent here in the first place. Star Vs is worse off without her.