I have a couple of groovebox-style machines (Korg Electrobe 2, Novation circuit) and they will let you select a scale and then the pads are locked to that scale. And then the internal synth and the MIDI out will just be in that scale/key.

I don't find it super useful-- I'm not bad at pianos/keys, but it does simplify note input quite a bit if that's what you're looking for.