Why does the back trouser panel get seam allowance for the fly?

Just to preface, I asked this question on the video comments at Vimeo. I have since re-watched the videos and gotten my answer. In the spirit of documentation for anyone else that gets this question, I am deciding to post both. My original question: I've got a question to the drafting of the backside waist. You say the finished line is made up of 1/4 waist + (3*3/8"+ 3/8" + 2*3/4") (i.e. seams, ease, darts). I'm scratching my head about the 3*3/8" bit. Two of those are the side seam allowance of the backside piece + seam offset into the front for easier pocket handling as mentioned in one of the videos. In the drafting video you say the final 3/8" is to account for the fly. I'm confused as to why we would need to account for the fly on the backside. Could you please clarify what you meant with that? Thanks!