Because you will sell more. It's that simple. Consider, for example, that a design you have created for a men's T-shirt may appeal greatly to a woman, or she may want a different colour or style, or even add her own text or put something different on the back. The same applies to items designed for parties, clubs, etc., where the same design with a few modifications may be ideal for other people, thus increasing the chances of a sale.

Always keep in mind that people like to add their own personal touch! Are you going to deprive them of this possibility if they really want it, and lose a potential sale? ;-)