In my case, it was because when I was shooting calendars, I also designed them and that allowed me to choose the image format shape, which was mainly 1x1.5 because I could see the final shape and frame accordingly as I shot. Even on Kodachrome there wasn't a lot of spare acreage to waste, and so it was really essential that I use every square millimetre to best advantage.
It would have been relatively simple to take thirteen shpots from a shoot and just cut them a little bit here, a little bit there, but sometimes those pics were also used backed up against one another as a grid on a single page, and so it was a nighmare to have to scale up different parts of each frame for those events, when it made sense simply to instruct the separation people on which shot went beside which, without having them try to use parts rather than the whole. Of course, doing ads was a different thing because much of the b/w work, for press use, ended up as cut-outs, and all the guys wanted from me was an image at so many inches from top of head to toe - they first did their own design/scale and messsing about on a Grant once they had the contacts.
Basically, I suppose, it's just nice to know (you hope!) that what you see is all you're going to get! Admittedly, that was where the 6x6 format had advantages: you weren't so tied in to the tight format shape, as distinct from just the tiny size.
So, really, it comes with the restrictions of the tiny size of the thing, which precludes wasted space and unnecessary enlargement, more than anything else.