A grit guard is a plastic circular grid type insert that sits at the base your wash bucket. It has four fins on its underside which means that when inserted it sits about an inch up from the bottom of the bucket. The reason you should use a one is simple; to help minimize the amount of dirt & grit particles being drawn over your cars paintwork during washing as these particles can cause scratches and swirl marks.
Using a grit guard helps to decontaminate your wash implement every time you dip it into your bucket before reapplying it onto the surface of your car. After cleaning each section you dip your wash implement into the bucket to reload it with soapy water and at the same time gently rub it across the surface of the grit guard. This allows any small particles of dirt and grit that have been picked up from the car to be released into the bottom of the bucket under the grit guard and safely away from the wash implement ensuring that they will not be picked up and redeposited onto the surface of your paintwork.
The fins on the underside of a grit guard also help to split up the water at the bottom of the bucket into four sections which keeps it calm and still. This means that every time your wash implement is dipped in and moved around, the deposited dirt & grit particles are not disturbed and remain safely at the bottom of the bucket. If a grit guard is not used in your wash bucket it means that every time you dip your wash implement in before cleaning another panel, any dirt & grit particles that have been previously released will be swilled around and collected by the implement and then redeposited onto your car resulting in more light scratches & swirl marks.