[somewhatshocked]

I've never dry started before but I wasn't imagining that there would be any appreciable amount of "rotting plant matter".

That's because there's not - you're right with your take that it doesn't accomplish much.

All a "dry start" does is allow some bacteria to start growing in the substrate. If the substrate is ammonia-rich, they'll feed on that. But it's still only the substrate and not the rest of the tank, hardscape or filter media. On top of that, water isn't moving around much in the damp substrate - there's no flow - so there's usually not a constant supply of waste for bacteria to feed on. More often than not, mold and fungus begin to grow because of stagnation.