Say you are doing an instrument training flight with a CFII and you are practicing IFR navigation. You will be taking off and returning to the same airport, but you are not certain exactly where your CFII might have you go. For instance, your CFII wants to see how you handle deviations in flight. Ths is just an example as I'm sure there are other reasons one would need to be IFR without a specific routing or destination. How would you file such a flight plan? Can you file an IFR flight plan without a specific routing?
Another reason I can think of would be a test or acceptance flight that needs to be conducted in class A airspace. You would have to be on an IFR plan to fly there, but your ultimate destination would be right back where you came from. You might not be able to indicate specific waypoints, etc., instead you would be requesting block altitudes, etc. if you need room to perform maneuvers.