Because HIV is a huge deal in the gay/bi hookup communities. They don’t force you to provide it, it’s just courtesy to your partners

It is protected, but you can disclose whatever you want to whoever you want. It's your medical data, you are not prevented from sharing it, healthcare professionals are prevented from sharing identifiable information including that.

Since HIV is such a big issue in the LGBT space, so much that many blood/plasma donation places won't even let gay patients donate at all, or if they e been sexually active within like 2 months, even though they obviously test all donations anyway. Being upfront with that information is important, so for a dating/hookup app meant for those users, it's essentially like having a space to put your height.

It's a hookup app, I think it's optional (I don't use it myself), but most people are going to want to know that.