The function of oil filters is to prevent contaminants which may be contained in the lubricating oil from reaching sensitive engine parts without restricting normal oil flow. The internal source of contamination include wear products from the rubbing surfaces on engine, blow-by gases leaking from the rings of pistons and de-gradation of oil itself.
The presence of foreign matter in the piston ring areas can seriously decrease their efficiency resulting in loss of engine performance. The most important job of oil is to lubricate the engine's moving parts. Dirty oil wears out these parts much faster.
Oil filters should be replaced along with the oil as the new oil can pick up the contaminants trapped in old oil filter and re-circulate them to engine's moving parts. Frequent oil and filter changes are an effective and economic way to increase engine life.