Let's say you want to bring the viewer's attention to a selected portion of your image. One way to do this is to use selective color, i.e. to allow that portion to remain in color while converting the surrounding areas into black and white. It's a fairly common effect, and some post-processing applications can do this effect automatically.
Here's a demo using Picasa, the free post-processing app from Google. In the original image, my wife's arm is much too bright and vivid, and is very distracting:
In Picasa, I applied the selective color effect (called "Focal B&W") to bring the focus on our daughter's expression:
I think it's an improvement from the original version. However, the selective color effect is very prominent, and may not look appealing after a while.
If you don't have the ability to control the opacity of the selective color effect, an easy alternative to soften the effect is to use warmify: