When defending, the most vulnerable area on the pitch is down the middle and close to the goal. The second most vulnerable area is if the opposition get the ball wide and deliver crosses into the most vulnerable area, centrally in the penalty area. The ideal defensive solution is to make it as difficult as possible for the opposition to be able to pass the ball into the vulnerable wide area. It can be too late if the ball has reached the flying winger and they are one v one with your fullback. That is why the role of the wide player with regards to their understanding of defending or filling space is so important. Their job is to get into space so that it is very difficult for the opposition to pass the ball quickly to their own wide player in an attacking area of the pitch. If they can position themselves so that it forces the ball to be passed to a wide player who is forced to drop deep to receive it or so that it is a risk ball that can be intercepted or has to travel in the air - passes in the air will always take longer to reach their target and to get under control - then they will have done an effective defensive job for their team.In the first example, the wide player has positioned themselves so that the opposition fullback in possession would have to take a big risk to try and pass the ball to their wide player and as a result forces the full back to pass the ball inside, which gives the central midfield player an opportunity to pressure the ball, backwards or to pass the ball long to the forwards, which is an easier situation to defend.
In this second example, the opposition have the ball in a midfield position and would ideally want to play the ball wide and forward to an attacking wide player. The responsibility of the wide player in this situation is to position themselves between the ball and the opposition wide player, making it a difficult and risky pass. They will need to have an open body position, so that they can see the ball and the opposition wide player, so that they can readjust their position if the opposition wide player moves to a different position.