As components in a transmission wear, the transmission filter sifts the debris. Transmission filters are only found in 5-speed manuals and need to be changed periodically according to the manufacturer's specifications. If it is not changed properly, then debris can get into the transmission fluid and possibly cause a transmission failure.
While most manufacturer factory warranties and extended warranties do not cover the cost of a transmission filter replacement, there are maintenance plans that will cover the cost. Also, if the transmission fails due to the lack of maintenance of the transmission filter, then most factory and extended warranties will not cover the cost of repairs.