An effluent filter keeps your drainfield clear of solids. They keep solids in the septic tank so they won't damage your treatment or disposal system. A filter is relatively inexpensive. Replacing a plugged-up drainfield can be very expensive.
All filters need cleaning eventually. It depends on several factors, including the number of people using the system, food preparation and housekeeping habits and the size of the septic tank. It also depends on whether a garbage disposal or water softener is connected.