Why did Police Officers used to carry a brass key?

Question

That included myself

Answer 1

To use the police telephone box.

Answer 2

In big cities they carried call box keys-I still carry a big brass key that lets me open traffic light boxes and over ride the automatic lights

Answer 3

It was a "skeleton key".  It got you into any public building in case of a robbery or something.  They are called "coppers" because they used to have copper badges.

Answer 4

So they could use police call boxes.