Reading through these Why I Write essays, I realize the importance of avoiding Boilerplate (that words sounds so weird if you say it a few times over). There is such potential for cliché, but I think that the essays I found avoided it pretty well using various techniques. I skimmed over a few essays by writers online and found two to focus on. Oliver Miller (who I think Casey mentioned in her comment too) narrates his process of writing his Why I Write essay. I think that readers can use this to understand his writing process more generally. Ian Welsh (another author) uses an “everything on the table” tone in his essay, and it matches his content. Both authors get specific when they talk about why they write, and I think that’s one of the keys to making this type of essay work.
The MiW Why I Write essays that I read used this same technique, among others. Keith Cline and Area Haider both document their development as writers. Area also focuses on her evolution as a reader. Both writers are very honest, and in that way, the style of these two essays (and the other two that I found online) mirror Joan Didion’s style. I think that this technique, which tends to evoke reader trust, is also really useful.