I had a look, (I think) they only breed animals that are part of the European breeding programme, which I think aims to build up a captive population of genetically diverse, healthy, endangered animals.
I think it's kind of like a back-up plan.
ETA: Just so that it doesn't seem like I'm against MW, I'm not, they seem like a brilliant organisation. I don't know how I feel about the captive breeding program, but no organisation does thing 100% how I'd want them to, this place comes pretty close. I've never heard of any other (non domestic) sanctuaries preventing animals from breeding to allow them to take in as many animals as possible - most love to breed the animals to create more to make profit from.
Any place that is not-for-profit and takes in animals that need lots of care and rehabilitation due to abuse is a great place, and a much better alternative than zoos/wild life parks with a couple of breeding programs/rescue animals and lots of animals bred in captivity just for the sake of showing them off to make money.