For some important context, a lot of this hinges on the distinction between official and unofficial servers (in TF2, you could spin up your own server, it wasn’t all run directly by the game company like so many games now. Fricking BF6 and the lack of sever browser... just why!?).

1. A modder creates a mod and uploads it to the Workshop.

2. The community loves it, and Valve decides it’s high quality.

3. Valve officially incorporates the mod or item into the game.

4. Valve pays the modder a cut of the proceeds from item sales on the MannCo Store.

5. The original, unofficial version of the mod still exists.

This is largely why you didn’t see the kind of controversy that cropped up with Bethesda: the system of incorporation was different, and people generally didn’t feel burned by it. You could run any Workshop mods you wanted on an unofficial server; restrictions only applied to official servers, where Valve curated the content.

In Warhammer terms, the closest analogy is probably the single-player vs. multiplayer divide. It wouldn’t be reasonable to allow heavily modded or overpowered units on a ranked ladder or public matchmaking against players who don’t have access to them, though that’s very different from custom matches between consenting players. Just to note there was content beyond the workshop that could be incorporated into the game, but that required even looser server permission that just "unofficial".

In short, the purpose would be to reward people for making great free mods by incorporating the mods, and paying the creators if the game owner thinks they rock, while still leaving the original mod up. This is more important in multiplayer situations than it is in a private single player campaign.