A high line shrinks the space opponents can play through, keeps the team compact and helps trap the ball in the attacking half when combined with a press.
It requires centre-backs with speed and good reading of passes, because any ball played behind exposes the defence to a one-on-one with the goalkeeper. Offside traps are often used to mitigate this.
High lines pair well with ball-dominant styles. Teams that sit deeper prefer a low block or mid-block.