A Diesel Particulate filter is designed to collect up exhaust deposits (soot) and this is trapped in the DPF till measured soot levels reach a certain threshold, once the manufactures threshold is reached the vehicle goes into regeneration mode to burn off trapped soot. Burning of this soot content leaves a small amount of Ash content every regeneration process, this Ash doesn't leave the DPF, a vehicle that has covered 80,000+ miles will have a high amount of Ash content causing all sorts of issues i.e more frequent regeneration processes/high fuel consumption, excessive wear and tear on vital engine and components more importantly the possibility to damage the integrity of the DPF structure due hot spots from Ash content.