The current purpose of this on the piano-roll is actually about quantizing and/or snapping notes that belong on the selected key and scale; that is, you then are only allowed to enter new notes that belong on the selected key and scale.
You may actually notice that there are a lot more scales that just Major and (natural) Minor on the scale menu or drop-down list. However, these have to be aliased or fitted to standard score key-signatures as major or minor scales, and these should be only meaningful to MIDI file import/export and for regular score editors (eg. MuseScore).
The current purpose of this on the piano-roll is actually about quantizing and/or snapping notes that belong on the selected key and scale; that is, you then are only allowed to enter new notes that belong on the selected key and scale.
You may actually notice that there are a lot more scales that just Major and (natural) Minor on the scale menu or drop-down list. However, these have to be aliased or fitted to standard score key-signatures as major or minor scales, and these should be only meaningful to MIDI file import/export and for regular score editors (eg. MuseScore).
Hope all this it's understandable :)
Cheers