Commander Kinsey wrote
Rod Speed wrote
Commander Kinsey wrote

Why did drainpipes used to have a box on
the wall, some sort of overflow if blocked?

Yep, and plenty still do, particularly with bigger buildings.

I fail to see why this helps. Let's say I try to empty my bath,
but the downpipe is blocked outside. I'd rather the bath
just didn't empty, then I'd investigate the problem. I don't
want the water to come sloshing out all over whatever is
below it outside, which could even be people.

It could make sense to stop water going back
the wrong way - eg. a 3rd floor bath emptying
into a 2nd floor bath and flooding the room.

That's the reason for it, to avoid that.

But there's no point on the highest floor,
which is where I've always seen it.

See link below for an example photo.
https://www.dropbox.com/s/1jry8zas14bmivc/box.jpg?dl=0