Clean Engine and Fuel System

Depending on where the deposits occur, you can have different mechanisms for deposit formation as well as different effects on the vehicle or engine. 

Different Types Of Deposits

Fuel system deposits are caused by instable fuel dropped sludge out of solutions and allowing it to accumulate on whatever surfaces the fuel contacts.

Deposits in fuel injectors form when trace amounts of leftover fuel inside the injector tip cook and oxidize after the engine shuts down.

Deposits in the combustion chamber and on valves form in a variety of fashions - pre-oxidized fuel not burning and cooking in the chamber, etc. Valve deposits are accelerated by fuel varnishes (unstable heavy portions of oxidized fuel) that accumulate as residue on the valve surfaces and provide a surface for the additional accumulation of similar compounds. Eventually you get a thick deposit that interferes with the most efficient engine operation.

Removing these deposits can go a long way toward restoring the performance of older engines to like-new peak performance. 

Removing Deposits

Deposits are formed by slow processes over time inside the engine. As such, removing deposits is a slow process involving the removal of the deposits in stages by a fuel-borne detergent/surfactant.  

As deposits in these critical areas are removed, it is said that the condition of the engine and fuel system are "restored toward new condition".  This gives the machine its best chance to perform at peak efficiency.  A new engine with little use is an engine that is most likely to operate at the maximum level it was engineered to perform at.