Why do some people have a keybase info in their HN profiles?
I understand that keybase is a way to encrypt things, but what is the benefit of having it in one's HN profile?
It's a proof that you control the profile. You claim on your Keybase profile that you control an HN (or Twitter, reddit, ...) account, and then you post a proof on that service to verify that you do indeed control it. https://book.keybase.io/account#proofs