Custom doors generally are made for two different reasons: to fit an existing opening in a door or a wall, or to certain size specifications, such as the opening being a certain size for a unique situation (
Giant breeds, dogs with mobility issues etc.).
Important Note: When having a door made to a rough cut opening in a wall; it is recommended that your hole in the wall is a minimum of 1/8" larger than the rough cut dimension chosen for the custom door.