Because you’ll sell more. It’s as simple as that. Just think: for example, a design of yours on a men’s T-shirt might really appeal to a woman, or she might want a different colour or style, or even add her own text or put something different on the back. The same goes for items designed for parties, clubs, etc., where that same design, with a few tweaks, could become ideal for other people and thus increase your chances of making a sale.

Always bear in mind that people like to add their own personal touch! Are you going to deny them this option if they really want it, and miss out on a potential sale? ;-)