Fuse box for one line. Fuses weren’t put in as standard when electricity was introduced. Many old houses just had live wires coming in with no breaker like today. For expensive electrical items added when there was no fuse, an electrician with install it with a dedicated fuse. If the electrical system has been updated where it enters the house, it may no longer be needed. However, if it is on a different circuit, it may be. Old fuse boxes were a bunch of replaceable physical fuses. Nowadays they are breaker switches for easy resetting and less waste.

Anyway, if whatever is connected to this gets a power surge, the fuse could trip and you would need to replace it. However it is not a plug, to add a different device or appliance, but just a safety pass through for the wire coming out the other end.