I mean, Skyrim is still entirely playable without mods, as shown by the fact that it was still a massive success on consoles and on release (when mods didn't exist yet).

Mods just make it strictly better. I love that the game can feel fresh because of the addition of new mods. And how I can increase the depth of the game with stuff like more enemies, more spells, and more equipment. Skyrim had good variety in the base game, but mods just made it unbeatable. And similarly, graphic mods can make this 12 year old game feel like a new release.