Seems to be a highly desirable vintage item. I assume because it is made for synths (therefore, it stays true to the sound but allows you to compensate for quiet or loud synths by evening them all out). Also, it ends up being perfect for getting a mix on your analog drum machine and spitting out a stereo track from the inputs (either to pop into a recorder, a DAW or another mixer). I’ve seen one of them once, and it was being used in exactly this fashion with an 808 and monosynth making percussive sounds to add some variation into the typical 808 kit. Merging two or three synths or drum machines into a single mixed mono or stero track is very, very useful.