In this excerpt from his Bitwig Grid modular tutorial series, the host patches an AD envelope into oscillator pitch and walks through the attenuator knob that controls how much the modulation signal affects the destination. Because the ear is most sensitive to pitch, it makes a clear testing ground for understanding how attenuators work in modular routing.
A key detail here: when pitch envelope depth is set to 12 semitones, you might expect to hear a full octave sweep, but the amplitude envelope's slow attack means the sound fades in after the pitch envelope has already peaked. Slowing the AD's attack stage reveals the full range.
The more transferable idea is what the host calls the attenuator test. When triggering notes from a keyboard, the perceived pitch with the attenuator open should match the perceived pitch with it closed. If it doesn't, the keyboard note no longer corresponds to the center pitch, and playing melodically becomes unreliable. It's a simple check that keeps pitch modulation musical rather than disorienting.