Because it's cool.

At some point, older devices will run into expired root certificates despite being still otherwise functional (can’t say how many apps would still work at this point but the browser might still render basic sites). If it is jailbroken, you can then technically try to update the certificate yourself.

I have an older 5S that my little one plays with. No OS updates for years, but still works with our public library app, so she could listen to audiobooks on it. We have a newer (I think) 7S Plus, but she doesn’t want to use it because she thinks the phone is too big… so I’ll be hoping for a jailbreak when the time comes for that phone, or some how try to convert her to larger devices as we’ve made the same leap.

Older iPhones will probably be sought out by retro collectors in the future similar to how old game consoles and computers are collectible now. I remember when you couldn't give away old DOS PCs and CRT monitors and now they are sought after by retro PC collectors. I could see the early skeuomorphic versions of iOS being sought after by collectors in the future. They'll want to sideload old apps and tinker around with the OS which is a lot easier to do with a jailbreak.