They provide a supply that bypasses the circuit's fuse. A malfunctioning device (not a dead short) might draw more current than its plug is rated for. E.g. a NE

They provide a supply that bypasses the circuit's fuse. A malfunctioning device (not a dead short) might draw more current than its plug is rated for. E.g. a NEMA 5-15 plug is rated for 15A, it needs to be installed with 14AWG (minimum) wire, rated for 15A at +30°C temperature rise over ambient, and a 15A breaker. Loads on the circuit must not draw more than 80% of the circuit's rated current continuously, 0.8 * 15A * 120V = 1,440W (12A). US devices aren't required to be fused, it's possible for one to malfunction and draw more than the 12A max, without being a "dead short". If that happens and it draws, say, 16A, and you've got a 4A solar panel on the circuit then the breaker will see 12A, the solar panel will see 4A, and the wires in the circuit will see 16A, exceeding their rating & posing a fire hazard.