major sketch pad is ideal for musicians with writer's block or knob twiddlers who easily get distracted by fine tuning soundscapes and ending up not getting any music done. No, this is not some pseudo-random-crap, you are in control of your music all the time when using the sketch pad. This is an arrangement utility rather than a music production tool.
major sketch pad is basically a step-sequencer combined with a chorder. Each time it gets triggered by a MIDI note the step-sequencer steps forward and each step can contain a new, different chord. The chords aren't locked to a predefined pitch. This means that you can try out chord combinations and see how your melodies will sound with different chords applied to them, or just jam away.
The main idea behind the sketch pad is to make it easier for people who are having a hard time coming up with songs to at least get a "rough" arrangement sketch that they can continue to work with. It is, after all, often the beginning of a song that is the hard part to come up with.