By the meaning, you can understand the what the mud guard does. Simply, those are made to prevent spilling of mud on the body while your car is moving. So! after you drive some distance, considerable amount of mud will be accumulated on all four mud guard of your car. Now the problems arise when you allow mud to be in mud guards for some long time because the mud guards are made from metal and metal will get corrosion due to mud and water in mud. Therefore if mud holds on mud guards longer, you are in trouble! You may need to spend some good cash in body shop for repairing the body. Also this will degrade the quality of your wheels.
To prevent corrosion, the solution is simple and clear. You just have to make sure that no mud lives longer on your car's mud guards. That's all! For that, you need to clean the mud guards thoroughly using a brush, car wash and water at least ones a week. But keep in mind that car lovers do it every day! I wish you are a car lover anyway!