Gravity Question...

Once "primary" fermentation ends, and gravity readings are stable, is there any harm for the beer to sit in the PF bucket, before getting bottled? In other words: what is taking place during this time?

Nope - no harm in letting it sit for a while. As for what it's doing: it's bulk conditioning - the yeast will clean up diacetyl and the extra time will ensure full conversion to alcohol (i.e. avoid acetaldehyde). Further, it's just generally aging, which allows flavors to mix and mellow like they would for any solution that you let sit for extended periods of time. Finally, it's clearing. Sediment and yeast floating in suspension will begin to drop out and leave your beer clearer.

People argue for secondary, but I think that it's really not necessary for a lot of beers, especially 'normal' ales. When I first got started, I thought it was an important step. Lately, a lot of the guys around here, especially ones that I respect and trust, have been advocating just going straight from primary to bottling. And I've tried it, and it definitely works, as far as I'm concerned, autolysis is a boogieman that a lot of homebrewers don't need to worry about. I've left stuff in primary on the yeast cake for months at a time with no ill effects.

Furthermore, skipping the 'secondary' vessel also takes out a step that can risk infection and oxygenation of fermented beer. And as TTB-J said, you still get your bulk conditioning without needing to move it around.