IN 1 - OUT A, B: you get the guitar signal to the 2nd device (combo, etc.) with one step. IN 1, 2 - OUT A: you use 2 guitars and you don't have to connect the cable, again just one "step" is enough and the second guitar plays. In both cases you have the option to set the volume of each path separately, e.g. an instrument with a single pickup has a weaker signal than your 2nd guitar with a humbucker, you can compare it using those knobs.