I asked this question about 30 years ago of one of the old-guard cigar chomping newspaper photogs that owned a pro-lab that I worked at. His answer was that as Roger stated, to cut down on what he called "frame flash", a bleed of light on the general easel area not part of the negative information.
I guess that the size variability issue is a genuine concern for masking by a carrier. I have filed out my beseler 35 carrier, even though FF ones are available. Seems like sellers of those think they are plated with gold. More often than not, I use the negatrans, which cuts more off the bottom of the frame due to the transport mechanism. A habit has been created to allow a little more frame space in composition.
A new issue I have run into is with the Mamiya RB backs. The newer SD backs have a slightly larger frame area than the Pro and ProS... It goes back to what old Cigar Bob said about composition, "if it's too tight it ain't right..."