The idea with the trench coat — as with most other things here — is to make things useful, but keep them simple. The first stage of development of the trench concerned its structure and shape — ensuring good balance at body front and back, and perfect pitch for the single-piece raglan sleeve.
Work on the storm-flap, back-yoke, two-way patch pockets — which also serve as the channel through which the belt passes — then follows. Up and up, development goes, through from the most time-consuming and fundamental stages, to the niceties of the dimensions of cuff-strap or pocket-flap.