Keybase desires to act as a keyserver and public identity record, keeping your aggregated identity, and a backup of your public and private key pair. The latter

Keybase desires to act as a keyserver and public identity record, keeping your aggregated identity, and a backup of your public and private key pair. The latter is (read: should be) encrypted and only accessible to you. They make that optional, though, and will happily keep track of only your public key. It is absolutely optional, and it exposes you in one critical way: You have to take Keybase's word that they cannot (and will not develop the ability to) access your private key, even though it's on their servers. I don't use that feature, because I can't see an advantage to it. I can keep a paper copy of my private key in a safety deposit box, if I'm worried about having a secured backup of it that's out of my hands.