Not everything has a 13 amp fuse in the plug. In fact most things have a 3 amp fuse unless they are kettles or something. There's also 5 amp fuses and it is possible (tho not that common these days) to have even a 1amp fuse in the plug itself.

Surely it is better to have a lower amp fuse tailored to the device, than a high amperage fuse that's happy to send 3000 watts into your dodgy mobile phone charger without fusing.