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.