The least active-looking player in a possession team is often doing something essential. Holding width is a positional instruction with a measurable effect on the space available in the middle.
Width forces a horizontal decision
A defence facing an attacker on each touchline must either send full-backs out to them or leave them free.
Sending the full-backs wide separates them from the centre-backs and opens the channels between. Leaving them free concedes the switch of play.
Neither option is comfortable, and the discomfort is the entire product of a player who has not touched the ball.
The central space is created by the players outside it
The half spaces between full-back and centre-back are where most creative players want to receive, and they exist only when the defence is stretched.
If wide players drift infield to get on the ball, the defensive line narrows and the space those same players wanted disappears.
This is why coaches often instruct a winger to stay wide precisely when he most wants to come inside.
Staying wide is harder than it looks
The instruction requires a player to remain uninvolved for long stretches, often with the crowd and his own instincts pulling him towards the ball.
It also demands constant repositioning relative to the opposition full-back, staying at the distance where he must be tracked but cannot be pressed.
Coaches reinforce it with pitch markings in training, because the discipline erodes quickly once matches begin.
The reward comes in isolated duels
A wide player held in position receives the ball with one defender in front of him and no cover, which is the situation a dribbler most wants.
Attacks that arrive via a switch produce these one-against-one duels repeatedly, and a winger who wins them regularly justifies the twenty minutes of standing still.
The pattern is often described as overload to isolate, and the holding of width is the isolating half of it.
Full-backs and wingers cannot both hold it
Two players occupying the same touchline waste one of them, so teams alternate: if the full-back advances, the winger moves infield, and the reverse.
The rotation is agreed by side rather than improvised, which is why a team may attack with an inverted winger on one flank and a touchline winger on the other.
Watching which player ends up on the line tells you how the team intends to attack that side, usually before any of it happens.