In an SSB/CW receiver, what is the purpose of the intermediate frequency (IF) filter?

In a superheterodyne receiver, the intermediate frequency (IF) filter is responsible for establishing the receiver's selectivity. Selectivity is the ability to separate the desired signal from other adjacent, unwanted signals on the band. Designing a highly selective, narrow-bandwidth filter is much easier and more stable when it operates at a single, fixed frequency. Instead of relying on the tunable radio frequency (RF) stages for this precise filtering, the receiver down-converts all incoming signals to a fixed intermediate frequency, where the IF filter provides the heavy lifting of isolating the signal. While filtering is involved in sideband selection at the transmitter, in a receiver's architecture, providing 'most of the selectivity' is the overarching and most precise description of the IF filter's job for both CW and SSB modes.