They don't always help, but sometimes they help a lot. They definitely keep your bike cleaner, leading to longer drivetrain life. A front fender keeps some water from being flung from the front wheel to your feet. A rear fender keeps water from being flung from the rear wheel to your back and butt. In a heavy downpour, if you're out for any length of time, you'll get drenched, but not all rains are downpours. In light and moderate rain, they reduce the speed at which you get wet. And the rear one definitely keeps dirt off your back and butt.