I've seen people mention three things they want a sidechain for: monero's privacy (without monero), liquid's opcodes (without the federation), and big blocks.

Personally, I think coinjoin provides sufficient privacy and bitcoin script is sufficiently expressive, so the first two aren't compelling to me. Big block sidechains seem like they could be useful in the future though.

Also, even though I don't personally want to use a monero-like sidechain or a liquid-like sidechain, I think it's important that they be available for people who do want them.

One usecase that wasn't mentioned yet is decentralized dns. There was a shitcoin, called namecoin, which did this but with a sidechain we wouldn't need this. Also interesting sidechain proposal is bitcoin hivemind.

Maybe It's just vaporware and out of my technical expertise but I like the Sia shitcoin concept. Never used it but it allows you to pay for decentralized storage. You just need to back up your seed and you can recover your data from all around the world like bitcoin. Could be very useful for a sovereign individual.