Ten pounds does not cover a missed appointment. A cut and finish is £46 and the chair sat empty for forty-five minutes — the tenner is not the point.
The point is the moment of deciding. Putting a card in turns I might pop in Thursday into a plan, and people keep plans. The shops I've watched try it see the no-shows fall off well before the deposits add up to anything.
It comes off the bill on the day, so nobody pays more. It just costs something to not turn up, which until now only cost you.