I think you’re right that designers can contribute in a variety of ways; and maybe we’ve gotten to the point where designers, UX/UI especially, are expected to deliver in a variety of ways. Whatever expertise or grouping a designer might identify with, writing comes in handy. Even for visual or brand designers, for instance, defining how someone perceives the design can be fueled by a thoughtful narrative.
There’s something to be said about the notion that designers come in all shapes and sizes, and by focusing on their inherent strengths, each designer can contribute in their own unique way. But for the reason I mentioned above, I have strong opinions about how writing can shape the quality of work for any profession, not just design.