The purpose of a fuse or circuit breaker is to speed up the current to slow down the current to stop current that is flowing too fast to stop current that is flowing too slow

Actually, the best way to put it would be if the current is too large or too small. 

Fuses and circuit breakers are both devices that "break" if the current flowing through them is too large.

A fuse is usually a thin piece of wire that will melt and break the circuit if too large a current flows through it. The thicker the fuse wire, the more the current needed to melt it and the higher the "rating" of the fuse. The function of the fuse is to protect the wiring and the equipment.

A circuit breaker is an electronic circuit that will switch off the current if it is too great. A circuit breaker is designed to operate much more quicker than a fuse and so the damage is less because the current is switched off in a much shorter time.

The purpose of a fuse or circuit breaker is to stop current that is flowing too fast.

Explanation:

In case of over flowing of current carrying charges, a circuit breaker or fuse breaks the current and protects the circuit from damage. A fuse is wire having low melting point placed in a circuit. When current is moving too fast i.e. in case of over current, the heat produced melts this wire breaking the circuit. Similarly a circuit breaker is a switch which opens up in case of over flow of current.