Note that keybase.pub was part of the Keybase filesystem since almost the very beginning. I'm not sure why OP posted it now.
What they did add was SSH server, banning, git hosting, and now bots to the chat.
As for what it is, their primary claim to fame is to solve the identity problem in the internet and secondary they solve the private key on multiple devices problem by having a smart way to have private key per device.
Once you have those 2 things you can build all kinds of stuff around that, and that's what they do. A well encrypted Slack is basically what they are.
I use it mostly to move files between machines or to host files I want to send to others. I also use the private git for backups for important folders.
The main thing that they are really missing in my opinion is being a IdP for OpenID connect.
Not saying it’s good or bad, but if you look at why people need trusted ID it might make more sense.
I transfer them into Bitcoin and sell them on Coinbase and receive the money through Paypal. I've received over 80$ for basically nothing as far as I'm aware. I'm happy with it.