Those just make an already good looking game look better, which is impressive but not nearly as impressive as when modders take a mediocre or even just bad looking game and make them look impressive.
One of the most striking example is easily Mount & Blade.
Here's what it looks like by default. I absolutely love this series but anyone with eyes that work can see the game looks like crap.
And that's an official trailer trying to make the game look good. Now here's the game modded up with things like Polished Landscapes, improved sky, and all other sorts of technical goodies!
Purdy...
Another good one is the STALKER series, which to be fair always looked good but with mods like Complete and Atmosfear look stunning! Probably the most beautiful environments and weather effects in any game I have
ever played, no contest. The trailers don't even do it justice, playing STALKER when a thunderstorm rolls around and you're stuck out in the dark is an experience like none other in gaming today!
