I think the target userbase for Keybase is nerds who found GPG too horribly unintuitive to figure out how to use it for themselves. I know I was like that for a while.
The other target userbase is people who want to verify GPG keys that don't have a web of trust. This is useful for, say, me - I'm not part of the Debian dev team or anything, so I don't have many people around who can sign my GPG key. It's nice to be able to publish a link to Keybase on my website and have people be able to be pretty sure it's me.
I actually really like the trust model that keybase.io uses. You control a lot of a distinct spaces on the web: github, twitter, email, personal domain, etc. It makes a lot of sense to have those reinforce a single public cryptologic identity.
The other target userbase is people who want to verify GPG keys that don't have a web of trust. This is useful for, say, me - I'm not part of the Debian dev team or anything, so I don't have many people around who can sign my GPG key. It's nice to be able to publish a link to Keybase on my website and have people be able to be pretty sure it's me.