Zoos take in animals whose habitats have been, or are being, destroyed, or that do not have enough food. Zoos help the animals by putting them in captivity, supplying them with food and a suitable habitat. Some zoos also take in animals who were orphaned as babies or were injured in an accident and can no longer fend for themsselves in the wild.
Additional Info: The San Diego Zoo has a center that is devoted to the welfare of endangered or threatened animals. They have been working for over 30 years. This center is called "CRES" the Center for Research Endangered Species. It is the largest Zoo based center of it's kind. They are working world wide (20 other countries) to improve breeding and genetic pools for those species standing on the brink of extinction. This is a multidisciplinary research team, and some of their goals are to, combine field, and lab research/studies to improve long-term captive and wild populations. To maintain and share the genetic knowledge and resources to support conservation. Also to improve the health of both wild and captive populations through shared disgnostics and research with other countries. To help revitalize ecosystems by restoring these species to the wild. There is much more detail about their goals and what they have achieved in the past 30 years. If you'd like more details see the site listed below.
This is just one Zoo's efforts to help animals in general. Most Zoos have some program in place to help with breeding, injuried, and/or rehab. of endangered or threatened species. Not all are as large or as estabilished as the CRES, but making an effort is the first step and that alone can help endangered species, or animals in general, by educating the public and planting the seed of compassion.