Because you will sell more. It's that clear. Think that, for example, a design of yours made with a boy's t-shirt may be liked by a girl, or she may want a different colour or style, or even add her own text or put something different on the back. The same for articles designed for parties, clubs, etc; where the same composition with some modification can be ideal for other people and increase the possibilities of selling.
Always keep in mind that people like to give their personal touch! Are you going to deprive them of this possibility if they really want it, and lose a potential sale? ;-)