Where does it come from? Where does it go? The reader may never know but you sure should!
Know the source and behavior of the magic in your world – not just to help with consistency but for inspiration. The generation or creation of magic may be indued with spiritual, scientific, or historical significance. Changes in the behavior of magic may foretell (or be the result of) profound changes in your world.
While brainstorming, you probably want to place limits on magic and consequences for its use. After all, if magic exists, then it coexists with other parts of your environment and therefore affects those parts. Whether you remove something that existed naturally or add something that did not exist before. Either action has consequences for the environment – even if this environment is in another plane.
In summary, if there is magic in your world, make it part of the world
Share how you integrate magic into your world.