The crankcase breather filter is good to buy because with four-stroke engines there are vapours in the crankcase that need to be removed. These vapours are mainly caused by gasoline/air mixture or combustion gas leaking along with the piston rings during the compression and working stroke of a four-stroke engine and ending up in the crankcase under the piston(s), especially in older engines.

In many cases, you have an original air filter housing where the crankcase ventilation is connected. If you mount a power filter on your carburettors, you will have to purchase a separate filter for your crankcase ventilation.