Their biggest advantage shows itself when you have an assistant to load them for you .
The inserts for the RB67 aren't as much help as some, because with their incorporated winder and counter really they have almost as much weight and bulk as an entire back.
The inserts for the Mamiya 645 are light, small and very quick to switch into and out of the backs. With matched plastic containers (also small and light) they allow you to pre-load a bunch of rolls and have them ready for quick use.
It can really help to have the exposed roll on an insert separate from the camera, because it allows you to separate the processes of sealing an exposed roll and continuing to shoot the next (note comment above about assistants).
One final point - if something goes wrong with a Mamiya 645 back/insert combination, like as not it is the insert that has the problem. Replacement inserts are easily swapped in, and relatively cheap.