You can use them for a bunch of things but the main reason I use them it is to allow me to access inputs and outputs without having to go tunneling behind racks and cases in an all-ready cramped studio in order to re-rout stuff. Instead I just move the patch cables around. If I suddenly decide I want a 32 band EQ in the signal chain... bang there it is ! 6r if I want a midiverb as in inset effect for the Focusright... bang, just move the patch cable over.

Another thing you can use them for is as ends (inputs) to a multi-core. This means that you can have rack gear a fare old distance away from everything else !