Doom shipped with a feature to launch with external wad (patch wad) files from the command line that could replace maps and textures. There's no reason to ship that apart from modding.
Not having to rebuild the executable speeds up iterations when designing a level.
Having the ability to easily have a single-level game is very useful during play testing.
Then there's PWAD ("patch" WAD), which are applied on top of the primary WAD ("IWAD").
There's no reason to implement support for that except for the ability to add modifications ("patches")
I don't know why people in the comments are speculating about this when it's well documented that this was the intention of the WAD system.