Turface: to retain moisture, and pleasing appearance.
Grit: since it doesn't hold any water, is grit used to balance out the moisture retention of the Turface?
Grit does hold water, but not as much as turface. So, yes, it balances turface (which, potentially, can hold too much water on its own).
Reasonably long lasting, holds water, might provide a very slow release source of N, and cuts weight and cost. Bark can be omitted if weight and cost aren't a factor. I recall Al recommending a 3:2 grit to turface ratio as a starting point, but you might alter it otherwise depending on circumstances.
bark: so people don't realize they are growing stuff hydroponically???
I never knew that bark can be left out. That would be great, it's the hardest ingredient to find for me. I ended up getting a few bags of repti-bark, not cheap. Thanks,