Because you'll sell more. It's that simple. Think about it: for example, a design of yours on a men's t-shirt might be perfect for a woman, or she might want a different color or style, or even add her own text or something different on the back. The same goes for items designed for parties, clubs, etc.; that same design with a few modifications can become ideal for other people, thus increasing your sales potential.

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