The transmitter is constantly issuing high power RF with continuous wave radar.
Electronic circuits have limited dynamic range, and this reduces receiver sensitivity.
Feed-through nulling significantly reduces the bleed-through signal, which can be used to increase receiver sensitivity in continuous wave radar.
The band-stop filter is generally wide enough to reduce clutter, in addition to the bleed-through signal.
Passive feed-through nulling is not used with CW radar altimeters because this eliminates the ground-bounce signal.