It's just a way of minimising your footprint. If you google me, and can't find what services I use, it makes it that much harder to try and find a foothold into hacking something, or building up a profile.
Then you wouldn't be using Keybase if that was part of your threat model - since a significant point of using it is to tie together and prove identities across some popular sites on the net.
This is why I use a different username on each site.
Some people use the same username on HN, github, reddit, gmail, etc. (and then they complain about internet privacy and being tracked.)