I am pretty sure it was for the photographers.
MarathonSport will do the photo things. They did the Kinglake ride as well. Back then we were given a number to be put on the helmet as well as one to be put on the bib (didn't say where, almost everyone had it on the back).
Thanks to the number on the helmet they could put all the pic of a certain rider together, and then offer them (for what I thought was a reasonable price) to the relevant rider only, without forcing every rider to go through thousands of photos (which is what happened with the pics taken at the Amy Gillet's GF).
I am sure that the need to put the number at the front of the bib serves precisely the same purpose. Wish they had thought of the number on the helmet, though ... way more practical, methinks.