PROS OF USING PASSPORT COVERS
They protect your passport from normal wear and tear, and getting liquids spilled on them. If you use your passport a lot, then this can be very good thing.
They help you quickly identify the passport in your luggage or bag. A dark blue cover doesn't always make itself known in a dark bag.
They look super fashionable or sleek if that's what you're going for.
They add extra travel organization. Add your important debit card, personal identification card, and passport while traveling.