When I owned an iPod touch, the advantage of jailbreaking was to give your device certain features that the OS just didn't support. (Hell, I remember early days when iOS didn't even have a concept of wallpaper, the background was just black.)
iOS has come a long way since those days so a lot of the most common use cases of the past are no longer necessary. But the principle is the same - the hardware is a general-purpose computer but the software is locked down unnecessarily in many ways. Jailbreaking lets you do things that are outside of Apple's box.
A much-needed advantage is also keeping old devices useful. Every Apple phone slowly outlives its supported software to the point where the official AppStore has almost nothing in it. Today, it's extremely difficult to find 3P apps that work with my current iPhone 7, which is not that old, because the ecosystem cuts off old phones very quickly.
I agree, and, if you have a developer certificate from Apple, the list of wants I have is ... not that long? zero? I'd love to be pitched on it. In practice, I wanted freedom, but didn't really want to do anything with it.
Let me try to pitch you:
The value of general computing freedom is like any other freedom: it is needed when it people are trying to express you.
And not just you. Anyone! So evaluating whether a freedom is necessary turns into a much more nebulous question of “would anyone really need this at one point now or in the future? Is this a thing we should protect in general, even if it does seem like we need it exactly now?”
For example, could Apple/Google at one point be strong armed (by any government) into including spyware onto all devices? And listeners to make sure no one ever says bad things about $DEAR_LEADER?
If the probability of yes is anywhere around 10% over the next decade, maybe it’s a good freedom to have.