Sphagnum - Loose or Packed Tight?

[Ray]

Whenever I use sphagnum alone as a medium, I tend to keep it a bit loose in the pot - packed enough to support the plant, but not tightly compressed like we see from large nurseries.

I have never understood the "physics" or "mechanics", or whatever you want to call it, of tightly packed sphagnum. (Those of you who know me well must understand my consternation at that!). When saturated, there is absolutely NO free air space in the pot, so it seems to be a great way to kill roots, yet the plants I acquire that's are potted that way seem fine. Is is due to the growing conditions they were in? I suppose that's it, as those plants do not remain "fine" if I don't repot them.

What is it that makes it "good" for some?

I have heard some speculate that the highly-compressed moss simply cannot hold much water, so between plant absorption and evaporation, dries out quickly, but if that's the case, what must the growing conditions be to result in that fast drying. It sure doesn't happen out on my deck.