From what I’ve read, it’s as much a marketing and protection game as anything else. Actual comparison tests of both DOT and Snell approved helmets show no Snell advantage — http://www.webbikeworld.com/motorcycle-helmets/motorcycle-helmet-faq.htm
Personally, one of my helmets has Snell approval, but that didn’t factor into the buy.
My best helmets are DOT approved. The day I see a helmet manufacturer publish data showing that their Snell tested helmet is statically safer in a crash, I’ll consider changing my mind.