I already know harmony and chord relationships and voicings, but my keyboard skills are lacking. I can play decently enough with practice, have progressed to a certain point, but the lack of skill slows me down when I'm composing and, frankly, can often stifle my creativity. This tool allows me not to get hung up on that and play chord progressions quickly and easily without needing to spend precious minutes, or even hours, practicing. It doesn't create for me, it merely helps me get the sounds inside my head onto the keyboard. Just like writing notes on a staff get the sounds inside your head down on paper.
Your comment, by the way, is exactly what I meant when I mentioned purists. Not everyone approaches composing in the same way, nor do they have to. Some of us are awash in theory, others of us rely on instinct and our ears. There's no right way to do it, and there's certainly no LAZY way to compose. Everything we use is a tool. The piano. Our DAWs. Our sample libraries. Our Vsts. Software like this. To accuse anyone who uses these things of being lazy, is a bit disingenuous.