It allows the roots to breath better, helps prevernt overwatering and air prunes the roots, so like in nursery pots they get root bound at the botton, but with the fabric pots, the roots hit the sides and they prune when they grow out,
so basically in a nut shell your plant shouldnt get root bound.
but they explain it right here if you would like to read it:
http://www.smartpots...-pot-air-prunes
http://www.smartpots.com/why-better