An example of why you need attenuators...or in this case, attenuverters on a DC-coupled mixer:
Let's say you want a VCF to sweep irregularly. Instead of having the same EG behavior each time, you could send the envelope and another envelope to such a mixer, then INVERT one of the envelopes. Next, rig up a VCA to control the level of the inverted EG signal before it hits the attenuverting mixer, and feed the VCA's CV with a slow LFO. This will cause a constant, repeating rise/fall in the inverted EG's input to the mixer. Send the output from the attenuverting mixer on to the VCF's cutoff CV. The result will be envelopes that have a weird behavior to them, and if you set the level-governing LFO to a period that's not related to the rhythmic pattern that the track your working on, the VCF will constantly output different but related behavior each time it's swept by the mixer's constantly shifting modulation result. This also shows why you've GOT TO have ample VCAs for instances such as this; for generative work, the "support" modules do more heavy lifting than one might imagine! And even if you're NOT doing generative patching, the ability to add a little "ear candy" like that is ALSO what those modules are for. In short, they're essential, and one leaves them out at their peril!