Depends entirely on the game.
For, say, the mess that is Oblivion, mods would be required for me to suck out any enjoyment from it at all. However in a game like Rome: Total War or Mount & Blade: Warband, the vanilla game can still stand up on its own, but both come with many additional "versions" of the game which are, more often than not, superior to the original, offer different things and are entirely free.
So for games like the latter, mods aren't necessarily important per say, but they sure as hell extend the life of those games and can offer a fresh experience to a game that may have become boring to you. Things like that.
For, say, the mess that is Oblivion, mods would be required for me to suck out any enjoyment from it at all. However in a game like Rome: Total War or Mount & Blade: Warband, the vanilla game can still stand up on its own, but both come with many additional "versions" of the game which are, more often than not, superior to the original, offer different things and are entirely free.
So for games like the latter, mods aren't necessarily important per say, but they sure as hell extend the life of those games and can offer a fresh experience to a game that may have become boring to you. Things like that.