I'm not sure why the OBC has never been digitized, while frame cameras like the UTAS DB/MS-110 and MS-177 series have been. It could be that the OBC has long been considered to be on the brink of retirement.
However, it is not unique simply because it still uses wet film. The continuous rotational scan covers a 40-mile-wide strip from the Firefly's operating altitude (72000-78000 feet) and it carries enough film to make the strip 1700 miles long. Resolution on-track was one foot, declining towards the edges of the track because of distance. I believe that the goal was to gather information on Chinese targets in regions where there were no good maps.