“Recently someone posted about their experience in determining the file structure of the Doom WAD file. How did you feel when people were discovering how to modify Doom, from building new levels, to changing the executable itself (dhacked) originally without any information from id? In your opinion, is the modding community a valuable place for creating future game developers?”
John Carmack Answers:
“The hacking that went on in Wolfenstein was unexpected, but based on that, DOOM was designed from the beginning to be modified by the user community.
[…]
I still remember the first time I saw the original Star Wars DOOM mod. Seeing how someone had put the death star into our game felt so amazingly cool. I was so proud of what had been made possible, and I was completely sure that making games that could serve as a canvas for other people to work on was a valid direction.”
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.
I don't think you understood the comment. The game supports two WAD files. One primary containing the primary assets and levels. This is the one they'd also be working on during development. No need to rebuild the executable when making changes to the primary WAD.
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.
That's a bit of a claim? Just because the data is separate from the binary doesn't say anything about the goal.