Ok - but, when do #lightning payments in a channel actually move on-chain? This was not explained here. I get the basics of channels now, and why I would need one - but this still doesn't explain how zapping works and why I would potentially need a channel for that. In fact, I am convinced I need none so far. O.o I'll just keep reading for now... maybe the rest of the docs resolve this, eventually.
Discussion
The money moves on chain when you open or close a channel. For the lifetime of the channel it just sits there in a 2 of 2 multisig, waiting.
Theres a new tech in development called splicing that will let you add or remove funds from a channel, but its not quite ready. I heard it will be done in 2 weeks.
Lightning channel balance moves on chain when that channel is closed.
However, there are these things called swaps - where people exchange lightning balance for on chain balance, thus they don't have to close their channel.