Small scale stuff you want on premise is a decent use case. The prime example being medical/dental. Imagine a patient with a crushed jarbone. You can model and print a custom prosthesis on premise and print it in maybe 16h-38h. Compare that to sending the job to some lab, custom molds being created and waiting for the item to be delivered (while the patient is waiting).

I know it's used quite a bit for custom tooling as well. Think special tools needed on an assembly line in automotive where pausing the assembly is very expensive or lead times for some custom tool to grab a wheel while working on it and whatnot are expensive.

I think one of the biggest success stories is cutsom made hearing aids. Return rate a lot lower, easier to iterate and produce the perfect fit etc.

Another example (maybe a bit more high end) is turning some item that is made out of many parts into a single item both for cost saving and other reasons. Iirc NASA uses ultrasonic welding (which is 3d printing of sorts) for this.