I purchase several sizes of roll film backs over the years though mainly use 6x9 or 6x7, rarely 6x6. I use whenever I want a larger format negative (anything larger than 35mm) but donít need 4x5, and shooting multiple shot close together and/or traveling and want to be on the light side.
Also, I choose if I donít need a smaller camera or the smaller camera doesnít have the capabilities that a press or view camera.
Large negatives are a dream to work with, which for me can be 6x6 to 4x5 in the darkroom. JMHO