Perhaps you manage a corporate campus with 10 buildings. Some of the buildings are unusual, like the parking garage, but most are the same: offices, lobbies, conference rooms, etc. All of these similar buildings share a common list of 20 request types.
Adding 20 request types to each building when you create would be a headache. Use default request types to speed up the process. Marking a request type as a default indicates that you want it to be added to new locations automatically when they are created.
Request types added by default can still be removed later, so it’s no problem if your new building is mostly, but not entirely, like the others. Use the default list as a starting point, and then customize it.