[wolfgang42]

Keybase is primarily about signing linked identities, not content--you don't (and, AFAIK, can't) sign individual tweets with it. Rather, you sign one particular tweet which links your account to a Keybase identity. Someone who knows you on Twitter can use that to verify your identity on Keybase, and then transitively on other services such as GitHub, HN, your website, PGP, etc.

In addition, it also has some additional features to make cryptography slightly easier for the layperson, such as support for PGP through a web UI: this is why you might want to upload your private key, though they make it clear this is a bad idea in high-security situations. For all of the core service, Keybase generates various 'device keys' which sign these identity verifications, the private keys for which never leave the users' computers.