So, you took a picture, but that one part of it that you really wanted to stand out... doesn't. Maybe the rest of the scene is too cluttered, maybe it's simply too big. Maybe you just want something that makes a striking impact, or maybe you want to further differentiate and alienate your subject from the surrounding scene. In any of these situations, color isolation can be a powerful tool, and certainly one worth experimenting with.
Color isolation can be used with, or without, different techniques for adding emphasis to the subject, such as vignetting, and it's worth mixing and matching different techniques to see what combination works best for a single image.
Also, be aware that color isolation makes an image feel very conspicuously processed, which some people find displeasing when compared with more natural-seeming photography. This by no means invalidates color isolation as a technique, however! Experiment and see what suits you.