To add: Many pre-90s buildings don’t even have circuit breakers, they have fuse boxes with fuse wire (different fuse to the one being talked about). Literally just a piece of wire that burns out at a certain current and breaks the connection. You “reset” it by putting a new piece of wire in.

The second fuse at the plug allows using a narrower gauge of wire in the device’s cord. Let’s say you have a lamp with a 3A fuse, the cord only needs to be able to handle 3A, so then it can be lighter and cheaper. If it had to handle the same amperage as the circuit it’s plugged into then it would be seriously impractical and expensive.

Of course there are modern ways of solving this but fuses are dirt cheap and already implemented.