For example, if you were establishing a mall area, then Fill and Forward might be good option, or splitting.
Another place I could see it being useful is InSitu resource processing to build a buffer on site. For example, I may convert all my Caterium into Quickwire at the source, but then a Fill and Forward storage fills before sending it on to my base. At any time, I know I have a ready stockpile of QW waiting for me that with a flick of a switch I could then order to send down the chain if I needed it.
It really depends how you play and organize, but for me, I really like having filled in-line buffers available at a moment's notice should I suddenly have a need.
Split filling just fills it up slower, while sending some items down the line.
As I said, you can accomplish similar with splitters and adding/removing conveyor segments, but this might be another way and add a smidge of intelligence to storage boxes that take advantage of the in and out conveyors, rather than just bulk storage at the end of a line.