They educate the mass populous and often, though they are not always (but sometimes are) completely helping preserve a species. For example, the Chinese softshell turtle was on the brink of extinction and the last known males and females were kept in zoos. chinese softshells are no longer alive in the wild, but the zoos keep the species alive by careful breeding and such. Many people also learned about the turtles at the zoo. However, one dangerous thing about zoos is that in times of natural disasters, they will often unintentionally introduce foreign species into the enviroment due to escaping animals and such (like in New Orleans... pets and other exotics will escape from homes as well so it's not just zoos).