Format to me is usually related to what size of negative you want, and most agree bigger is probably better for printing.
But what is interesting to me more is aspect ratio and why people prefer, what they use.
I never liked 35mm in the early 70s and when I moved to Hasselblad in 1976, I never looked back. I used is almost exclusively professionally and personally for over 30 years. I dabbled with 4x5 in view camera form and also with a Littman but 99 percent of the time is was 6x6'
Strangely though I never printed a square print. Every print, and there were thousands, were the standard 4x5, 5x7, 8x10, 11x14, 16x20, 20x24, 30x40 etc.
When I retired the Hasselblad and went digital I started using full frame 35mm and for the first time printed 16x24 and 20x30.
Now I see them framed, I like the color pictures, but when I see the black and white, black frame and white matt, placed next to my old analog versions, for some reason I like the old format better I think.
The traditional sizes seem like a better aspect ratio for some reason.