Microcopy is at the heart of user experience. It allows users to understand your product and molds how they think about and interact with it. Writing microcopy is about understanding people, what they want and what they expect, and using that to design an experience that will have them coming back.
There is strategy involved, too. Microcopy can reflect a personality, such as you've already seen on this website, or it can be strict. A good product writer will write microscopy that fits the product's strategy.