It's the combustion process, inside your engine, that is responsible for producing soot deposits in cylinders, pistons, particle filters, EGR valves and turbos.
Clogged parts prevent the engine from functioning properly. If carbon deposits (soot) are not removed regularly, faults can occur and the costs can soon mount up (expect around 350 euros per EGR valve and up to 2000 euros for a turbo...).
What's responsible in more than 75% of cases?
A clogged engine.