G'day Stevo, I guess the best reason I can come up with for knowing numbers is purely from the organizing point of view. Eventually we will need to know how many people plan to be there and where and how they are going to get there eg: The 4wd owners might want to come in from the south and traverse the rough stuff first. It helps to know who we are waiting for at the rendezvous point.
It also helps the organizer have a good idea of just how big the M&G is going to be by knowing numbers and this makes planning activities a little easier like bushwalks, sight-seeing etc. I am sure Lucky_Phill will add to this.
Regards,
Poodroo