Steven Watkins wrote

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

Because that's how your stupid regulatory
authoritys decided it must be done.

Can't be for safety

Fraid so.

- 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.

Its done like that for the other situation, so you
don't get out of the shower dripping wet and use
the switch dripping wet and get a shock that way.

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

Correct.

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

But that's one that can be safely used dripping wet.