Why is butter incorporated into the dough last when making Brioche? I made Brioche for the first time tonight using the Rich Man's Brioche recipe from Reinhart's Bread Baker's Apprentice.

The recipe basically leaves out the butter until the very end when the dough is fully mixed and hydrated. Only then does the recipe require the butter to be slowly added into the dough tablespoons at a time using a wooden spoon.

I am usually used to creaming the butter at the very beginning or using melted butter in the wet ingredients and then mixing it with the dry ingredients. Incorporating the butter into the dough at the end using a wooden spoon took quite a while and was a pretty good workout for my arms. 

I took a look at other Brioche recipes on the internet and pretty much all of them add the butter into the dough at the very end. 

So my question is why is the butter incorporated only after the dough is fully formed? 

What would happen if I were to cream the butter with the sugar(small amount of it) and egg at the beginning before adding it to the sponge and dry ingredients?
The reason is that butter can inhibit gluten formation.  It 'coats' the proteins that would form gluten.  You knead the dough first to get gluten, and then add the butter afterward around the already formed gluten.

You can add it earlier, you just end up with less gluten and a more tender dough.

Creaming isn't usually done with bread, as its purpose is air bubble formation.