Why A roofing filter sits right after the first mixer, at the first IF, so it narrows the signal bandwidth before the later IF amplifiers, AGC detector and product detector ever see it. Strong signals a few kHz away from your frequency get knocked down there instead of overloading the downstream stages, which is exactly what blocking (and close-in intermodulation) dynamic range measures. The narrower the roofing filter, the better the receiver holds up on a crowded band during contests or pileups.
Watch out The sensitivity claim is the common trap: the roofing filter comes after the front end and mixer, so it cannot reduce front-end noise or improve noise figure, and its insertion loss slightly hurts it. The low-Q claim is backwards too, since narrow filters are high-Q and ringing is a side effect to be managed, not a benefit.
Roofing filter = the roof over the IF: it keeps loud neighbors out, improving blocking dynamic range, not sensitivity.