Some people don't understand the value of tie-in media. The truth is, a good tie-in can improve the media franchise as a whole. Tie-in media almost always receive better sales than if that same tie-in had been released without the brand name attached. It boosts awareness of the franchise as a whole. But even with all of that, it's just fun to learn more about a world you already love.
Although an adaptation is not, strictly speaking, tie-in media, it does have a similar idea behind it. It is taking a popular media franchise and translating it for use in a another medium. This can be almost a direct adaptation, but it can also be an extremely loose adaptation, drawing inspiration from a single character, a single plot point, or even something as simple as the relationship between two people.
Literature is one of the most common forms of tie-in media. It often supplements a visual franchise like a film, television show, or video game franchise. Literature tie-ins include novels, short stories, websites, and comic books. Each one of these has something new to offer to the franchise and can really help develop the story world. Some of the literature tie-ins act as great stories by themselves, even if they are part of a larger whole.
Visual tie-ins are some of the more enjoyable stories, though they usually cost a lot more than literary tie-ins. That is why you will only see a visual tie-in to a literary franchise if it is extremely popular. Harry Potter is a good example. However, that doesn't mean that visual tie-ins are not common, especially in visual franchises. Marvel, in addition to their films, also has short films and a television show for example.