Spinners are on birds to direct the airflow into the cowling for proper cooling. On the higher performance birds, they're essential.

BTW - spinners are also used for other things, such as the tip of the spinner being used as the datum on Cherokees.

[Linked Image from animatedimages.org] [animatedimages.org] [Linked Image from visitedstatesmap.com]
Imagine a united world.
Join the Popular Front for the Reunification of Gondwanaland.