Does it reduce wind resistance? Because, how to explain this to you simply... you're falling. And not in the Olympics. Does it matter if you shave a tenth of a second off your free-fall?
Is it to protect your head? I think, could be wrong here, that it might be a waste of time considering the situation will not be triumphed by a few inches of flexible, cone-shaped plastic.
Is it so that we can tell where your head used to be before you fell? See, now this sounds plausible.
To keep your head from swelling when you brag to everyone that you went skydiving? Could they be more cocky? This might actually be marketable.
A skydiving instructor wore it on a dare, and someone mistook it for safety attire. Who really wants to put that on? People who want to laugh at you want you to put it on.
I don't know people... what's your theory