The primary goal of a motorcycle helmet is motorcycle safety – to protect the rider’s head during impact, thus preventing or reducing head injury and saving the rider’s life. Some helmets provide additional conveniences, such as ventilation, face shields, ear protection, intercom etc.

The origins of the crash helmet, according to Wikipedia, date back to the Brooklands race track in early 1914 where a medical officer, Dr. Eric Gardner, noticed he was seeing a motor cyclist with head injuries about every two weeks.

He got a Mr. Moss of Bethnal Green to make canvas and shellac helmets stiff enough to stand a heavy blow and smooth enough to glance off any projections it encountered. He presented the design to the Auto-Cycle Union where it was initially condemned, but later converted to the idea and made them compulsory for the 1914 Isle of Man TT races, although there was resistance from riders. Gardner took 94 of these helmets with him to the Isle of Man, and one rider who hit a gate with a glancing blow was saved by the helmet. Dr. Gardner received a letter later from the Isle of Man medical officer stating that after the T.T. they normally had "several interesting concussion cases” but that in 1914 there were none.

In May 1935, T. E. Lawrence (known as Lawrence of Arabia) had a crash on a Brough Superior SS100 on a narrow road near his cottage near Wareham. The accident occurred because a dip in the road obstructed his view of two boys on bicycles. Swerving to avoid them, Lawrence lost control and was thrown over the handlebars. He was not wearing a helmet, and suffered serious head injuries which left him in a coma; he died after six days in the hospital. One of the doctors attending him was Hugh Cairns, a neurosurgeon, who after Lawrence’s death began a long study of what he saw as the unnecessary loss of life by motorcycle dispatch riders through head injuries. Cairns’ research led to the increased use of crash helmets by both military and civilian motorcyclists

Gardner pushed for his invention to be used by racers in the Isle of Man TT, the premier motorcycle race of the day, and was successful; organizers of the race made head protection mandatory the same year, and an immediate reduction in concussions followed. The first ever "helmet” immediately proved its worth. But civilians still balked at the idea of wearing one of the odd-looking shells while riding; most riders used no head protection at all. Some opted to use padded leather caps similar to football helmets of the day, but these were marketed as a way to hold the hair in place and keep road grime out of it. A skullcap and goggles was about the most protection you’d see on any rider in the early 1900s.  

According to bikebandit.com, his research led to him lobbying the British military aggressively for some improvements in motorcycle safety, especially since in those days, motorcycle mounted "dispatch riders” were still used widely for reconnaissance and communication. Cairns’ efforts paid off in 1941, when the Army ordered all military riders to wear head protection, a "helmet” made of rubber and cork.  

In the 1970s and 1980s, full-face helmets came into their own, with improvements in rider comfort like flip-up and tinted visors, later followed by the introduction of modular helmets, which combined the convenience of ¾ helmets with the safety of a full-face.