Keybase solves a (to me) nontrivial problem: How to bring private keys into social media. Just a silly example: You don't use the same private-public key exchange in Whatsapp as you would use for your emails, or to sign your packages. It's a bit of the now infamous Dropbox situation: Most people can sign things with private keys and properly keep track of it, but they don't get around to doing it. It's only critical cases where the use is common (like signing packages). It took a long time even for HTTPS to become standard practise, though I guess the situation with your browser is a bit different.