To counter oppression. It's not so much "pride" in the sense of an achievement, but in the sense of not feeling shame for who you are, and publicly asserting that. It's also valuable from a visibility standpoint.
I'm reminded of when people complain about Black History Month and ask, "When's White History Month, eh?" And the answer is that every month is White History Month. Because white people have more or less wiped black people out of popular history. This erasure is just as present for LGBTQ+ people.
It's not.
(If you're American, I don't know how you can live in the US and not have noticed this)