It is almost always better to get your lighting correct when on set. You will want a properly exposed and composed photograph to be certain. However, there are times during post production when you may want to polish the image. In some cases you may not always have the type of lighting on location to bring your concept into play when you have camera in hand. Photoshop can be helpful in some cases to produce some very creative effects not only with portrait photography, but with product, architectural room shots, and many other areas.
The "Render lighting" under the filters menu in photoshop has some very powerful options that will allow you to dramatically alter how an image is lit, Options include spot light effect, triple lighting, omnidirectional lighting as well as several others. Each style of lighting can be configured to suit the taste of the photographer or creative team.
The link below is a "Lynda Tuitorial" which gives a good general discription of this filter. After clicking on the link, simply click on the play arrow in the center of the screen enjoy the presentation.