There is no definitive answer to whether or not helmets prevent head injuries. However, many experts agree that they can help reduce the severity of a head injury if one is sustained.
Helmets are designed to protect the wearer's head from impact by absorbing and dissipating the energy of a blow. They work by spreading the force of an impact over a larger area, which reduces the amount of force that is transmitted to the wearer's skull.
While helmets cannot prevent all head injuries, they can certainly help reduce the risk and severity of them. Wearing a helmet while participating in activities where there is a risk of head injury is always recommended.