In Lightroom 4 there is a ‘noise’ slider under ‘development’. I have never used this slider. When working with RAW files from my Nikon D5100 or Canon S95 there is the inherent noise (grain) of the sensor. But as I have a large archive of old material shot with simple compact camera’s in JPG I have discovered the use of ‘noise’. A jpg by default has artefacts. If you reprocess those jpgs’s and for example want to ‘sharpen’ the picture, those artefacts will be sharpened, which is highly undesireable. Adding some noise will ‘smooth’ the artefacts. After that you can apply some sharpening, etc. The resulting picture will be more pleasing. Yes you have introduced some ‘graininess’ but, esp. in black & white, it can add to the atmosphere. Just don’t overdo it.
Shot with Sanyo Xacti J2, 6 mm (38 mm on 35 mm scale), f/2.8, ISO 200, 1/250 sec, JPG (2009)