A motorcycle helmet lock is useful for two main reasons. It keeps the helmet protected against theft when the rider is away from the bike, and it gives the rider freedom from having to carry the helmet around wherever she goes.
Though the protective capacity of a used helmet cannot be correctly estimated and the risk of the strap being worn out or the inner structure being cracked are quite high, an unlocked helmet dangling from a bike is still a magnet for thieves. They can strip it and sell the visor, the chin strap and buckle. Alternatively, they can put it on online retail sites that sell used products and require very little information. The safest way to protect a helmet is to lock it to the bike.
Motorcycle helmet locks also have the benefit of giving the rider one less item to carry. This can be especially useful in formal scenarios such as a wedding or job interview or even when on a picnic, when carrying a heavy and unwieldy helmet can be bothersome. With a strong and tamper-proof motorcycle helmet lock the rider can leave the helmet securely fastened to the bike for as long as required.