A well tuned hydraulic system gives trouble-free operations. A few regular problems that crop up are leaks in the system leading to starvation of oil and air bubbles in the system due to ingress of air.
Hydraulic oil and systems are very sensitive to contaminants. These systems need to be maintained in very hygienic conditions. Filtration is one of the modes of keeping the hydraulic fluid clean.
The linked article covers the modes of filtration with their benefits and drawbacks.
Clean up hydraulic circuits
Phillip Johnson, PlantServices.com
Selecting the proper filter leads directly to greater uptime, lower maintenance expense and longer machine life.