Why? To offer more products with my designs and photos. For instance the Zenfolio store has the best printing for images that I can offer to my customers, but their choice of greeting cards is dismal. They do offer greeting cards but they are not folded cards ... merely single sided photo cards like the ones that people send out at Christmas. Don't get me wrong ... they are wonderful cards, but if you want folded cards with envelopes in a variety of sizes along with some other products then the Zazzle store is the one to look at.
The images in Fine Art America can also be ordered as cards. You can compare prices and image selections to fit your needs.