I remember back in the day when people would “Jailbreak” iPhones, but never really picked up on what they were doing other than that it let them do stuff that those of us with “non-jailbroken” iPhones couldn’t do.
Are they just booting another OS, e.g. android? Also: why haven’t I heard of it in a while? Is it not possible on newer iPhones?
I just want to add jailbreaking iOS is still very much a thing today. I’ve been doing it for several years. You can take a look in r/jailbreak, as there is not a comparably-active community in Lemmy.
As for the main reason, as others have stated, is to have root-level control to your device, which alows you to install alternative stores, which besides providing apps not sanctioned by Apple, also provide tweaks to alter various things in your system (i.e. alter your homescreen look and feel, change your statusbar color and icons, notifications behaviour and style, etc etc.).