Fuses and Circuit Breakers
Two different cables: 3 core and a 2 core
A 2 core cable would be connected to a double insulated appliance. This is something which does not have a metal outer casing.
A 3 core cable would be connected to a metal casing appliance. This is because a 3 core cable has an earth wire.
A live wire is very dangerous as it could give an electric shock. Fuses and circuit breakers are used to prevent severe electric shock in the case of a fault where the live wire breaks and becomes in contact with the outer casing.