If you're PC is up to it, much, much better graphics even without mods.
Being able to use the console to fix bugs, cheat, test or do whatever you like.
Being able to download thousands of truly incredible mods, adding to the replay value tenfold.
Having the option of creating your own mods if you wish.
Being able to modify the INI files to change a multitude of settings.
Can take screenshots and upload them to the Steam Library if you wish.
You get to try out the beta patches, and the stable patches are released 1-3 weeks earlier than on consoles.
You can use the Steam Overlay to browse the internet whilst ingame.
You have the option of using a keyboard and mouse or an Xbox controller natively. You can also use a PS3 controller with a bit of tweaking.
Can use a monitor, a large tv and can play in 3D with a 3D monitor/TV.
Being able to tweak, modify and change the game to your hearts content.
Just a few advantages things worth mentioning. So yes, it's definitely worth it
