When does a catcher back up first base?

A catcher backs up first base primarily on plays where a batter hits a ball that is fielded by an infielder, particularly on ground balls or throws that may go awry. This is crucial to prevent runners from advancing if the throw to first base is errant. Additionally, the catcher should back up first base on any play involving a potential pick-off attempt, ensuring there's a safety net in case of a missed catch. Being in position helps maintain defensive integrity and minimizes scoring opportunities for the opposing team.