Staying clean and dry.
Imagine being sprayed with water and mud because your car didn't have mudguards ..
-well , mountain bikes or road racers don't come fitted with mudguards either.
Having mudguards, chain-guards and skirt-guards make cycling much more practicable as you won't have to worry about getting wet, dirty or having the leg of your trousers or your frock caught up and ripped by the chain or the back wheel.
mudguards, chain-guards and in many places with skirt-guards for ladies.