Long story short, they reduce the severity of whiplash, and prevent you from eating the steering wheel or dashboard, or in the case of side curtain bags, from whacking your head against the glass.
They’re not perfect, as they can actually cause some injuries, and I’m not just talking about the Takatas, but in general. The propellant and fabric can cause chemical and abrasion burns even in the best case scenario. But I’d rather have a minor facial abrasion or burn than a major concussion.
