Many times people just starting to make textures will leave their objects in pristine condition. This makes the objects in question look as if they were taken from a CAD program or The Sims.
Adding dirt, grime and everyday wear will greatly increase the believability if your textures.
In both versions the viewer knows exactly what it is but the version on the right looks much more real. The reason is that you will never find objects in the condition on the left. I have taken the wear to the extreme but it demonstrates my point.
The first thing I do for a texture is to lay down the materials that it is made out of. This is the stage to the left. Now it "reads" as the item in question albeit a perfect one, straight from the factory.
Then I find rus, grunge, slime, dirt, graffiti, chips, drips, gouges and layer them on over and over. A good rule of thumb for environment stuff is this: If you have less than 3 layers on any given part you probably have not pushed it far enough.
One great benefit of using so much dirt and layers to modify the texture is that no single layer has to be very good quality. Some of the layers on the drum were taken from palletized gifs but you cannot tell because of how many layers are interacting.
If you had not seen these images right next to each other the first version would probably pass as a good as it is but when compared to the gritty version, its perfectness is exposed as bringing down the believability.
