Why do houses have a switch to turn the shower off, either a cord on the ceiling or a switch in the hall?

Can't be for safety - if you're in the shower and get a shock, if you've managed to get out to reach the switch, you've got away from it anyway.

Can't be to isolate to work on it, there's a fusebox for that.

Don't need to turn it off when you're finished showering, there's a switch on the shower itself.