I'd say the main reason to spend the money is simply the sheer number of mods available. For example, say you want to improve the graphics to bring it closer to today's graphical quality. How many mods do you need to install? You need mods for models, textures, animations, and you need them for characters, enemies, animals, buildings, terrains, etc. Then you also need ENB or Community Shaders, and all the required mods. That's easily tens or hundreds of mods just to make the game look prettier. Are you going to investigate what all mods you should install, and then download, install and configure everything one by one? With a modlist, that's 10 euros and one click, and you get more than just prettier graphics.