Carpenters- wood versus all purpose drywall fasteners?

I'm repairing a wooden dresser, and this is making me curious- on one type the threads go all the way to the top, on the other, they don't. I've heard that wood screws leave some of the area unthreaded "to allow for expansion from humidity"... But when I think about that practically, I think isn't the head of the screw holding the wood tight anyway? How can any expansion take place? The kid at the hardware thought it was just cheaper for a company to make screws that don't have complete threading. So, is there a real function for this, or not?