Hat 'n Hoodie

I don't understand the exact logic behind the wearing of both a hoodie sweatshirt as well as an additional hat. In my thinking, the hoodie alleviates the need for additional head apparal, but I'm certain there is some perceived coolness factor that I am missing. I just think it's cute when he accessorises ;)
I certainly have no complaint when he picks to bring along a book rather than PSP...