Not only do I not find these labels senseless, but I also find them useful. If I start to catch feelings for somebody, I want to know if they even have the potential to like me back. If I like a guy, but he’s gay, I want to be made aware of that as early as possible so I know to back off. Being aware of other people’s sexualities allows you to respect them, and know if it’s okay to pursue them.
Also, I don’t know many people who orient their lives around their sexualities. I’m bi, but that’s never directly affected my personality or behavior. I just happen to have a more varied dating pool than I once thought. Sexuality is certainly very important like you said when it comes to relationships, but I’ve never known anyone to make a non-relationship-related decision based solely on their sexual orientation.