So here's a hypothetical scenario:
I am playing a modded solo game with myself, and my friend Matt invites me to play a D&D adventure.
In order to join Matt's game, I would have to de-activate all of my mods every time I wanted to play, then turn them back on to play solo.
This is a huge barrier to modding, because correctly remembering and manually changing the settings on each mod for each of my active campaigns, online games, and other campaigns is an enormous hassle.
My save game or game client should have a list of mods when starting a campaign or joining one, and if I have them available, they are activated and loaded.