RF Filters are usually used as pre-selection stage before LNA of transceiver RX path. It suppresses signals at other frequencies becoming blockers. This increases IP3 and EMC robustness of the transceiver. Other effects are also increasing of Signal-to-Noise Ratio (SNR) and suppression of mirror channel in heterodyne receivers.
RF Filter are also placed at TX to suppress harmonics of power amplifier and fulfill reglementation of FCC for out-band harmonics.
There are following flter types:
Broadcast TV, FM, CATV