They don't "work" at all – if the rider doesn't go down. This is not a trivial objection. Note that it is fundamentally the same objection of the not-sick to being told they must wear a Face Diaper . . . on the basis of the assertion that they might be sick.

Some will say that "just in case" is a good enough reason to wear one. Well, maybe. But also maybe not.

A helmet by itself – as is all the law generally requires – only "works" to protect the head  – in a limited way.

The rest of the rider's body – assuming all he's wearing besides the helmet (and his ordinary street clothes) remains unprotected. Certainly, the head is the seat of cognitive function, without which the rest of the body doesn't function. But helmet-wearing doesn't prevent spinal cord injuries, compound fractures or other blunt-force trauma to the body, which can be crippled for life even if the helmet did protect the rider's brain. 

And that is uncertain, as wearing a helmet does not immunize the rider's brain from devastating injury, depending upon the severity of the impact. They merely serve to keep the skull intact, making the body more presentable for viewing in the Slumber Room. 

Some helmets – perfectly legal, in terms of meeting the letter of the law – are useless even for that. I refer here to the "beanie" type that provide head protection comparable to the protection afforded by a third of a life-vest.