I now make a point of reading over art descriptions for commissions & informing my clients if they are actually looking for an illustration instead of a map. What's the difference & how do you know when to get which?
When you want an illustration: If what you really want to convey is a vertical space, a sense of depth, interesting concepts or ideas, etc. If what you're really trying to show is an interesting idea such as a city built into the side of a mountain, a village residing among the tree tops, etc. but don't need to give specific directions or distances. In other words, a narrative aid.
When you want a map: To show the location of several Points of Interest within a specified space or to give specific directions to get from one location to another. Maps are wonderful for invoking a sense of exploration but they are also meant to be tools for your readers/players. They show distance, give a sense of direction, offer a route of travel that needs to be provided in order to accomplish something or to tell a part of the story. In other words, a story-driving tool.
What if you want to both show specific directions AND a cool concept? Then you want a map. Any time you need to direct characters from one point to another, you need a map. Any time you just want to show a cool land feature but don't have specific points in mind, you want an illustration.
When to get a map with inset illustrations: A map that shows both the road map of that city built into the mountain and a sideview illustration is a great combination of both a narrative aid and a story-driving tool. However, keep in mind that you should expect to pay an artist for both the map and the illustrations needed, meaning, it might be even more expensive for the map.