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.
thanks for all the replies
specific to the fuji gx680
looking at the storage box it looks like you can't store an insert
can anybody confirm either way
Is the box 'Green Cardboard' or 'Dark Grey Plastic'
If Green Carboard it is for the original MkI and MkII backs and there is no problem having the film loaded (see image below)
If the plastic box then that is for the newer IIIN backs and as the backs were multi-format (via a masking system) and could also be used for 120 or 220 by changing the insert and the box was designed only for storage, not as a means of having film pre-loaded. A newer IIIN back comes with a pair of inserts one for 120 film one for 220 film and you used the insert required at the time.
The older backs were format and film size dependant so you could buy a back for 120, 220 film and 6x7 as well as 6x8 format (I have a 6x7 as well as x3 6x8 backs) but as far as I know the 120 inserts can be used in any (pre IIIN) 120 back.
Green Boxed insert with Tri X pre-loaded
Thanks for the detail description
It's a mk3 insert in the plastic box
Dose look like you can't pre load as there are plastic vanes in the box that would get in the way
I could always cut them out I suppose but seems strange that they are limited in this way