[SifJar]

I believe it is becoming less and less useful. To begin with, it was the only way to get any third party apps on iOS devices. Later it became the only way to have custom wallpapers, multitasking, better notifications etc. Obviously all these things have now been added officially, and in most cases the official implementation is more user friendly and more integrated.

There is still increased ability for theming with jailbreaking, and some unapproved apps such as emulators etc. as well as a few general tweaks such as allowing extra apps in the dock or allowing the home screen to be rotated on iPhone, but the benefits are slowly decreasing as iOS gets improved. But there is still benefit to it for some.

It's the same case with rooting Android devices; at a time it was almost essential to address the shortcomings of the OS. Now, it is barely even required for most users, due to the improvements of the OS. (stuff like taking screenshots, making backups of app data, tethering etc. used to require root on Android; now it's all baked right into the OS)

I'd say it's noteworthy though that these features may never have made it to their respective official OSes had they not been so popular with jailbreak/root users, so it's an important part of the OS improvement process in my opinion.