Some say there is a use for everything, and yes, we can use 'Fibers' effectively in photoshop. The above image shows a mock castle wall behind the model. If you look closely, most can tell that it is not really a stone wall but a backdrop. We can improve upon the effect with Photoshop Fibers.
Old castles lead, minerals seep out of the blocks, and mold grows along walls. Below is an initial image that
We see a dialog box open up with what looks like black streaks. You can change how close the streaks appear to each other as well as the density of the streaks by adjusting the variance and strength. In this case, I left both sliders at their default values.
Selecting OK, will produce black streaks over the entire image. To get something usage, we can:
If we look once again at the image on the right at the top of the page, the background looks a little more like actual brick work. We could continue if we wish working on the wall however the 'Fibers' features gives makes the image look significantly different.