I started with two ADSRs. As my system grew I was looking at the options for more envelopes. I realised that in my patches I often used one of my ADSRs as a decay only. This module was the obvious choice.
In just 8HP Doepfer have squeezed in 4 decay envelopes! I rarely make a patch that doesn't use this module in one way or another. It can have a really sharp decay so can make snappy rhythmic sounds fantastically well. I often use it triggered by some different rhythms from a logic module to make drum machine sounds. In conjunction with the A-151 Sequential Switch you can make a varying decay for a filter cutoff, for example. Mix two together one triggered by a sequencer clock and the other by one step in the sequence and you can generate an accent.
Put the CV through a slew limiter and you can add an attack portion to the envelope too. Jumpers on the circuit give you options for looping too.
This is another module that you'll keep finding new ways to use. And it's got flashing LEDs to let you know what it's doing.