Aside from the practical reasons to travel light, to avoid being burdened physically by too much stuff to carry or aesthetically by having too many choices to deal with practically in the field, I've found that when I revisit some place over a period of years, I like the images to have some unity of form, so they can eventually all look good next to each other in a portfolio or an album or a show. You don't want to end up at the end of the process, where you're trying to assemble a portfolio of 10 images, and you've got nine square format B&W prints that flow and one terrific 35mm color slide that you would just hate to leave out, but that doesn't fit with the other nine, and you're wishing you'd been carrying the 6x6 camera at that moment.

It doesn't have to be too restrictive, but, along the lines of, say, for a certain place deciding to shoot only medium format color transparencies, or maybe a 35mm folder with a fast lens and fast film for night and a medium format folder of the same era with medium or slow speed film for day, or only 4x5" B&W, or only negs big enough to contact print. I can use my other cameras for other projects, but setting a few formal boundaries at the beginning of the project can help later.