• To get more depth of field.
  It's more economical...
  Less volume (2 rolls or 10 sheetfilm holders)
  When you ran out of sheetfilm.(Backup)
  You will use the best part of the lens(Image circle)
  Larger variety of films available.

Actually there are two types:
# 1 Roll film backs are "graflock" style.
# 2 Roll film backs are "Springback" style.

First of all, for our Speed Graphic we can choose from 6x6 or 6x9 rollfilm holders.
These holders have to be used on Graflock backs. For the tubular finder you need also the matching mask.
If you do not have an international back you can use rollfilm holders from Calumet or the like. The holders will slide in like the cut-film holders.

There are quite some models available on Ebay.
Most common are the regular ones - 6x6, 6x7 and 6x9
THe other series are called RH/** made by Singer Graflex. Instead of a knob to wind the film, these holders have a yellow or red handle.

Roll Film holders can be divided in two groups i.e The ones that slip under the GG screen of a springback like the cut-film holders and the ones that need to be locked with the clips of the Graflock(International) back. The Calumet roll film holder is belonging to the first mentioned group.

A more sophisticated (means higher price) back is available from Linhof.
The Super Rollex backs are available in 6x7 and 6x9. There is a back for 6x12 also, it is called Techno Rollex.
And then there is the Cine Rollex (56x72 only). These are made for 70mm double perforated film (Spec. 475). Very few films are available in this form:!

In case you start wondering if these transparent backs are lighttight, Yes they are...
This is a technique from the analog days and it is called: Phantom Image or Fantoom aufnahme. Two images were made on the same piece of film, differing in exposure time 1 or 2 E.V
Nowadays we can do this superimposing much easier...

Extremely versatile is the Sinar Zoom cassette.
Formats: 4,5x6 - 6x6 - 6x7 - 6x9 -6x12.
Different formats can be chosen on the same film.
Sinar also has a 67 and 69 rolfilm holder in its program. Holders are designed to accommodate all backs that can handle the regular sheetfilm cassettes.

From China....Lately I've noticed more and more photographic goodies from China.
From China means reasonable prices compared to Sinar and Linhof, but is the quality really is up to par?? Google for DaYi to get all the info you need.

A real gadget for Linhof devotees. Hard to get at a reasonable (Graflex-budget) price. Worth looking for...In between, just cut a 4cm from your 4x5 prints.

Finally this GuoHua adapter, looking very well made, reasonably priced.
Till now I could not find any user reports.

Please note there are many more brands making rollfilm adapters in various formats. Just to name a few more: ShenHao, Wista, Horseman, Canham and Alpa.