Protection. That's the whole purpose behind wearing a helmet. You wouldn't wear one otherwise.
Broad brush - two kinds of head injuries: penetrating/points hits (i.e. you fall and your head hits a rock) and diffuse axonal injuries (i.e. shearing of neurons - "Shaken Baby Syndrome"). Virtually all helmets protect largely the same on the first kind due to statutory standards in place. The second kind (and maybe the overall worst to recover from) is just now being addressed with new technology in helmets. The most prevalent of these technologies is MIPS (
http://mipshelment.com). You can find this technology in POC, Scott and a few others. MIPS protects from an obtuse hit the head that causes the rotational acceleration on the brain that causes the shearing.
Having lived through a serious sports related TBI with a family member, we researched this heavily.
This excellent article from Bicycling magazine explains it very well from a cycling perspective.
SENSELESS | Bicycling Magazine
Because of the statutory requirement for specific testing and protection, virtually any helmet made today will provide equivalent protection to every other helmet made for cycling. The test is a drop from a specified height down to a hard floor of a specified hardness so that the helmet lands on it's top. It's about the least representative test one could ask for compared to the actual use of the helmet in practice. So you might as well pick for esthetics, fit etc... The only exception to this is MIPS and some manufacturers that are (finally) advancing the protection beyond the mandate statutes and addressing DAI injuries.
I've talked with the engineers at a couple of the helmet mfgs and I've corresponded with the MIPS people in Sweden. I'm also an engineer and I'm pretty convinced that this is a serious advance in helmet technology at long last. So much so that this year I replaced all our skiing helmets and we're in the process of replacing cycling helmets as they become available. My son just got a Scott MIPS helmet. Very nice, great fit and it's got MIPS. He's the one that suffered the TBI, so he's first because he's the most vulnerable.
Finally, helmets need to be replaced much sooner than 15 years. As you drop them and knock them around, you begin compromising the protection by actually crushing (and therefore losing) some of the lining. Virtually all bicycle helmets are the "one and done" type of protection so you have to be careful of this over time. As well, the materials can lose some of their elasticity and become less protective.
J.