And more to the point, it's enough to put many many more people off seeing the doctor, even if they could easily afford it, because they think "it's probably nothing so it's not worth it". Inconvenient times/rationing by queueing has the same effect but on a slightly different group of people.

A prescription charge doesn't work in quite the same way because if you are getting a prescription then you already know you need it, unlike going to the doctor.