Here's the doc on that: https://keybase.io/docs/server_security/merkle_root_in_bitco... . Since users on Keybase append their "announcements" (proofs, username announcement, etc.) to a public signature chain which is verifiable, the Bitcoin merkle step is designed to prove that Keybase is (a) showing everyone in the world the same thing, and (b) not omitting anything from the ends of people's chains.