Probably the autosave feature in our medical eForm application. You might think that doesnt sound unnecessary, but it was built to solve a problem that shouldnt have existed in the first place.
Autosave was put in place to rescue as much data as possible in the event that the app randomly crashed while a paramedic was filling in a form.
So in one sense it was a necessary feature because the app was so unstable. But on the other hand, if the app was built properly it would never have been needed.