setting the lash in 8 3/4

I've never took before measurements. whats the point, its all coming apart and will be setup once its all together. maybe someone explain this reason ??

because as the gears break in (assuming it was done correctly and no abnormal wear) the clearance will grow, and continue to grow throughout its life. if you turn around and setup a set of gears that were running @ .012" back at .006" its not going to be happy.