Human beings are pretty fragile. You've probably broken a few bones and had some serious scrapes over the years, but there's one body part we can't mess with – the brain. The whole essence of who and what you are is tied up in this one squishy blob. Give this three pounds of gray jelly anything more than a firm tap and it swells up, breaks down, and turns to mush. Then you're history.
Luckily, we've all got a reasonable tough carrying case in the form of a skull. The brain floats in a bubble of fluid inside this solid bony dome, meaning you can take a certain amount of abuse without it flaking out. But it's not too hard to exceed the protective capabilities of this stock setup: if you fall and hit your head on concrete from as little as one and a halve meters you can sustain a head injury bad enough to kill you. Bust an arm, leg, even smash some ribs, and you're still here. Bust you're brain and you're visiting the maker.
So it's clear that the most important protective item you can wear is a helmet. By adding a solid extra layer of impact protection to your head you stand a much greater chance of surviving a knock on the head. And if you're going to engage in a risky activity – such as riding a motorcycle in Thailand – then a helmet has to make sense. Therefore I still wonder why I can see so many people riding motorcycles without helmets...???
The first thing to realize about a helmet is what it can't protect you from. And what it can't do is let you head-butt a car or wall at 120km/h and survive. Helmets aren't there to protect you from high-speed impacts with other vehicles or local concrete – they just can't be made big enough to absorb all the energy in such an impact. All the helmet can do is defeat more moderate hits such as landing on your head after a highside or dropping off the back of a wheelie.
The guy who was wearing the orange helmet in the picture was riding not over 80km/h, he is very lucky to live and tell about his accident....