Usually functionality is given to the user for free as an update to the base game (like the latest Warfare additions), with the new cosmetics being the focus of the DLC. Like
Darkfyre99 says, it's a bit more nuanced than that, because some "cosmetics" do, IMO, give gameplay advantages. For example, there's no advantage that I'm aware of using sci-fi thrusters vs vanilla thrusters, that's just cosmetic. But the spotlight, which is part of the Warfare 2 DLC, does give an advantage, because it's a combined camera and light in one block that can be set to track you as you walk around your base, which IMO is a huge gameplay advantage. Sure, you could probably (maybe, with scripting) build such a system yourself, but it would be a lot of work. I bought the Warfare 2 DLC primarily for the spotlight. I don't mind, because the DLC is cheap and I want Keen to keep supporting Space Engineers, so I'll pay my "dues". I get a
lot more from a Keen DLC than I do a paint job or ship kit in Elite Dangerous for the same price.