When photographing people you generally want the person to be the main focus of the photograph.  They may be cases where the person is not the main focus but that is another story.  If you are taking someone's portrait or headshots of a model you want them to be the main focus.  Often times your client will show up in some type of distracting clothing.  Take a look at the images below.

 
Notice how much your eyes are drawn to the shirt in the first picture and away from the subject.  In the second picture the subject stands out more.  You could make it even better if the clothing, in this case the blue shirt, blended with the background more.  That would further focus the viewer's eyes on the main subject.

 It's a good idea to recommend clothing choices for your clients before the shoot.  Solid colors and long sleeves for adults are advised. Your subjects don't all have to have matching clothing but the colors should be harmonious and not compete for attention with the subjects.