Through accreditation work done over the decades by organizations such as the Association of Zoos and Aquariums, zoos and animal parks have been held to ever-evolving standards of animal welfare that reflect improving knowledge of animal habits and habitats. The number of animals stuck in concrete pits has declined over the years as more and more effort is taken to re-create – as best as is possible in captivity – the aspects of life in the wild that give animals satisfying lives. Terms like "ecological psychology" and "landscape immersion" reflect an interest in climbing beyond the very base of Maslow's hierarchy, a psychological theory of human motivation put forth by Abraham Maslow, and trying to satisfy some of the higher needs of animals and observers alike.
Outside pressure (from films like "Blackfish") can play a positive role, too. SeaWorld's response to the film "Blackfish" has been combative (this lively back-and-forth between SeaWorld and the filmmakers gives you a sense of the feisty tenor) but knowledge that outside observers care deeply about the welfare of captive animals – and are willing to write or make movies about it – can only up the pressure and stakes on organizations to get it right and keep moving forward.
2. They help animals in the wild
Animal parks (and their donors) represent motivated organizations that have a vested stake in preserving wild habitats and populations of animals. As a counterweight to poachers and developers, they're a critical voice on the scene.