To add to what everyone has already explained here, I think its the opportunity to create one trick pony sidechains so a zcash-esque privacy side chain, a side chain to hold domain names, a side chain is EVM style (which you could just use RSK) and then linking them all together with Lightning/Thunder if you don't want to move back to the main chain and wait 3 months or you can't find someone to sell you a certain dc coin for another

Anyway I just think like we don't even know if these uses for a side chain are needed, I mean isn't that what altcoin chains are for? I don't think I've seen an altcoin chain yet where I was like man if this was on Bitcoin I'd be stoked