I think the OP explained it pretty well but the TL;DR is that it allows you to try out FCMP++ without having to sync a testnet node and keeping up with the stressnet - a Fullnero node is synced in less than a second.

Also like I said, I think merge-mining is cool and I'd love for Monero to become similar to Litecoin in that regard.

As for your second question, no one should use it over Monero - that's why it requires you to run a Monero node to be able to mine it - but if you're someone like me that likes to play around with different cryptocurrencies: This offers FCMP++ and AuxPoW via P2Pool which is a unique combo.