First, because other players are reluctant to pass to a teammate who is tightly marked for fear of losing possession, and second, because if the teammate does receive the ball, he is immediately restricted in his options because of the man-to-man marking.
A group or unit in the team may use specific tactics to gain or take away an advantage. For instance, players in the central midfield unit may mark their immediate opponents on a man- to-man basis. Opposing midfield players find receiving possession of the ball and operating when in possession of the ball difficult under the tight marking circumstances. By applying this tactic, a team may reduce the number of times a very skillful opposition “creator” in midfield both receives the ball and creates problems. First, because other players are reluctant to pass to a teammate who is tightly marked for fear of losing possession, and second, because if the teammate does receive the ball, he is immediately restricted in his options because of the man-to-man marking. Some teams that play with a sweeper and man-to-man defense also mark on a man-to-man basis ahead of the rear defenders and sweeper and so mark midfield opponents individually.