The purpose of a full broadcast is when you are first setting up a federation. You can also use this functionality to resolve a "stale-server" status.
Resource-heavy Operation
Because a "full broadcast" is resource-heavy, you should avoid repeated Broadcast clicks whenever possible.
To trigger a full broadcast from a specific Access Federation source, navigate to User Management | Access Federation.
Select the source you wish to broadcast from, and from three-dot menu on the right, select Broadcast.
You will be asked to confirm the full broadcast and resync.
Click OK to begin the broadcast or Cancel.