It's a way for mod authors to get paid with Bethesda taking a cut too. Not sure how much they take, but they are generally regarded as crappy mods. Think there are also free mods you can get as well for consoles through creation club but better mods are available on the Nexus.
I see it as a game of wasted potential really and bethesda's usual attitude of letting the community doing the bug fixing for them which is a right pain. Also some of the graphics from a distance look absolutely terrible even on Ultra. It's an okay game, I can play it and enjoy it. But it definitely isn't their best game ever released