Walls often have some dirty rain streaks, we can use puffy clouds to approximate this kind of dirt too:
boost the Y axis scale and you should get something like shown in figure I. it is nice if you consider that it only took 10 seconds to set up the surface and render it! ;) if you dont like you could ever bake the texture and paint over it to fix and give the shape you want to the streaks. (yeah that will be ever the best way to do things, you can add lots of stuff with procedurals but it will never be like painting your own dirt maps)
These are just some uses for this very versatile procedurals. Summarizing they are very useful when you need to add tiny details over a surface. They're also great to "hide" seamless textures by randomly give variation to a otherwise repeating, ugly pattern! their lacunarity is great because you can control the smoothness of the effect and adapt it for various pourposes. as already said the most common uses are to give subtle variations to your textures and to dirty up things too clean.
Dont forget the power of falloffs!! using falloff on some axis you can distribute various kinds of procedurals/image maps layers and make them blend toghether in a very effective way.
just remember that values for the procedurals on this tutorial are almost lightwave defaults, you have to play with them to have a less obvious look!
That's all for now I hope that you learned a bit more of procedurals working/uses, they're a speedy way to achieve this kind of effects but use them wisely and in a subtle way, they can enhance the look of your textures or make them worse if used badly!
for any question/comment/flame feel free to contact me: [email protected]