What is the
point of a
two-way zipper?

A two-way zipper may seem futile, but it does indeed have its advantages, for it lets you to unzip the top or bottom of your garment while leaving the other extremity closed.

Consider, for example, when standing in a winter storm, you are looking for your car keys in the side pocket of your suit jacket or pants. Rather than unzipping your jacket completely, from the top, to reach said pocket, exposing your person utterly to the cold and damp, you need merely pull upwards on your jacket’s lower zipper pull, while remaining comfortably warm and protected!
More importantly, a two-way zipper helps you respect a longstanding rule of elegance stipulating that a jacket should never be closed all the way to the bottom such that it cuts the figure’s line in two. Thus, just as elegant men never button the bottom button of their suit jackets, they quickly learn never to zip their outerwear jackets all the way to the bottom. The two-way zipper: a combination of practicality and sartorial sophistication!