With nothing between developer and fixer, it means that you are carrying a lot of crap into that fixer. It means that the fixer exhausts faster, and the chance of retained chemistry goes up!

As format size goes up, then a stop becomes more critical for uniformity. You must stop development quickly or you will get some sort of blemish on the final image. This is particularly important with LF and ULF films and very big enlargements.

PE