Why would a recipe say mix all ingredients, then add egg and mix again?
There isn't really a technical reason for this in most cases.  It is for the convenience of the cook, to get things mixed thoroughly with a minimum of mess and hassle.

However, with meat loaf, once you add the egg, it gets even more sticky and messy. 

When I make meatloaf or meatballs, I actually do it in three stages:


Mix all the onions, herbs, spices salt and so on into the breadcrumbs so that they are uniformly distributed (everything except the meat and eggs).
Mix that with the ground (minced for you UK types) meat, loosely breaking it up and tossing with my fingers
Add the eggs and mix again briefly just to incorporate.


The only purpose of this is to easily get thorough mixing, with minimum messiness, and minimum additional working of the meat.