Fullback Roles and Responsibilities In Soccer - Cover the centre backs

The fullbacks are usually the fastest players in the defensive line, this means that if a long ball goes over the center-back head the full-back must be prepared to track back and cover the space behind the center-backs. Depending on what side the ball is on the fullback will be the deepest lying player in the defensive line. For example, if the ball is on the right-hand side the defensive line will shuffle to the right, this means that the left-back will be the deepest lying defender and will be covering the space in the middle, in behind the center backs. Because of their speed fullbacks are great at stopping counter attackers, they are able to match the speed of the attackers and delay their immediate threat to the goal. This gives enough time for recovering defenders to get back in position and gives them the numerical advantage when defending. https://soccersourcecoaching.com/how-to-be-a-successfull-fullback-in-soccer/