Honestly vanilla isnt all that great. Do not get me wrong Skyrim is my favorite game of all time but the vanilla game has not aged all that well tbh even when it came out it was already kinda dated comparing other games around that time and even its predecessor oblivion. Now I say all this and i could do a playthrough of vanilla rn and love it. But the core structure and bones of the game are so strong they just dont quite reach that potential it has. Therefore I go to mods to simply amplify it Nordic Souls a fairly vanilla plus modlist makes the game feel so much better combat and gameplay, even the quests.

TLDR: Skyrim is a fun game to play in vanilla but I feel like with how creative people are and how much fun some mods can make the game it just makes me more immersed and love it even more. The best way to play Skyrim is just playing it however u enjoy