ControlMaster with ControlPersist reuses one TCP connection and one authentication for subsequent sessions to the same host, which removes seconds of handshake latency from tools that open many connections, notably Ansible and git over ssh.
The common failure is 'ControlPath too long' or 'unix_listener: cannot bind to path: ... too long for Unix domain socket'. Unix socket paths are limited to roughly 104 bytes on macOS and 108 on Linux, and the traditional template of %r@%h:%p easily exceeds that with long hostnames or a deep home directory. OpenSSH 6.7 added the %C token, a hash of the connection parameters, giving a fixed-length path: 'ControlPath ~/.ssh/cm-%C'.
Operational notes. The directory must already exist; ssh does not create it. A master connection outlives the command that started it for the ControlPersist duration, so config changes do not take effect until it exits or you run 'ssh -O exit host'. 'ssh -O check host' reports whether a master is live. Multiplexed sessions all die when the master dies, which makes a flaky bastion a single point of failure.