[Keybase] What is it useful for?

Their file system is useful for sharing encrypted files.

Other than that it's pretty much a key server with account verifications, so it's good if you want to sign or encrypt documents and give the recipient an easy way to confirm the key identity.

I was my understanding that it was a mainly used as a means of verifying that User-A on website One is the same person as User-A on website Two. In other words, I thought it was being used to prevent impersonation.

It's useful for, amongst other things, making it harder to know if someone is the sole owner of their private GPG key.

Don't use it.