Why should oil be filtered during operation?

Just like blood in the human body, oil circulates throughout all the components of a lubrication circuit.
During its journey, it becomes loaded with various pollutants. Contaminants fall into 3 categories: mineral – most often silica (earth, sand, etc.), vegetable or synthetic – such as pieces from the seals, flakes of paint, fibre from cloths, etc. and metallic, such as steel, iron, yellow metals that are the result of wear or running in.
A large particle in the wrong place at the wrong time can cause a major fault in the circuit. This is why a properly configured filtration system is required in order to maximise the reliability of lubricated equipment.
How effectively this filtration works depends on how effectively the level of cleanliness required or recommended by the hydraulic circuit's designer is adhered to, depending on what it is used for and its design.