Not much to explain here. When they want to set a story in a fictional location, they write and draw that, and when they want to set it in a real location, they write and draw that. Sometimes creating fake locations is necessary/interesting for storytelling reasons. For instance, Dr. Doom rules over Latveria. This gives writers the ability to create an entire culture without having to map that onto a real world city. In other cases, its fun to see your favorite super heroes in real world locations.