Storage containers are great as buffers, and as overflow/endpoint pools. But There are 2 other cases I would like to see.
"Hold" toggle. Set container to fill first, and only supply overflow to the exit. This enables an in-line depot for storing frequently used goods without adding splitters and spaghet.
"Round-Robin" push pattern. Select one (or n) item of each, in a rotating fashion to supply to the exit. This would help manage flow control on multi-item belts & splitters.