So say you're a guitarist or keyboard player and pick up a little tiptop or Synthrotek eurorack case in order to make use of the great effects modules in eurorack land?
Mutable Clouds, one of the best granular synthesis entities ever, has its own pre-amp and volume gain, not too hard to get a guitar signal in and then start FSU right?
Well in reality, you'll need a Mono big to little adapter for the guitar, and then you're dealing with two Mono little jacks at output, and adapters sticking out of sockets are notably frail, one wrong elbow could break the lot!
So this module lets you take a Mono signal into your clouds (and another Mono in for another module?) and then you can return your signal to a stereo standard jack, which you could maybe break out to an RCA cable or whatever you want with simple leads.
It is also the easiest way to let your eurorack speak with the outside world, and can be used for CV gate converters that use large jacks, and many other situations.
Get one in your modular plan from the start, it's a no brainer, don't use 75p adapters.