And of course I'm obligated to link to Getinlost in Colorado, who posted a haiku about fenders.
Fenders enable me to ride my bike without getting the stripe up my backside. Fenders keep my shoes from getting soaked through except in the worst downpours. Fenders help keep muck and grit and salt away from the bottom bracket and other drivetrain parts. Fenders prolong the life of exposed cables. Fendered cyclists might attract wheel suckers.
What are some other benefits of fenders?