As long as can be remembered (exotic) animals have been collected to satisfy man's curiosity for beautiful, dangerous, extraordinary, and human-like creatures of the animal kingdom. At first it were the world's rulers who, using their power and authority, collected animals and established menageries, either by having the animals sent from occupied territories or by receiving animals as gifts in return for favours. Later, the rich and wealthy could afford to establish menageries of exotic animals, collected by eccentric explorers, for their own pleasure. Occasionally the naive citizens were allowed to view these collections. Fortunately, this gradually changed when travelling menageries came into existence and zoos were founded which allowed the common people to visit their grounds. At first one day or morning a week, Sundays for instance, but soon this was increased until the animals were on display for everyone, year-round.
Next, came the understanding that animal welfare was impaired when animals were housed alone in small cages, and continuation of wild catch of endangered species would be counterproductive in the end. This caused a shift in zoo policy worldwide, knowing that breeding success of endangered species in zoos relied on good animal husbandry systems and that diversity of animals in the wild was at stake. Maintaining the diversity of the animal kingdom became the main driver for many zoos and a requirement to be member of a continental or global zoo organisation (WAZA). Therefore, their mission is focused on sustainability in general, leading to objectives concerning welfare and care of animals, biodiversity conservation, environmental education and global sustainability.
This can be recognised in changes made in the management of modern zoos. The animals are offered an environment which is more or less similar to their natural environment, particularly to allow development and expression of natural behaviour. The result lead to enclosures that provide animals the opportunity to isolate themselves or hide, which is natural behaviour too and makes them less visible sometimes. This requires understanding of the public that primarily come to see the animals, and therefore needs explanation, guidance or a different approach. After all, visitors bring in the ever so needed money, thus a decreasing number of visitors is undesirable. Careful balancing between these two "benefits" is therefore essential.
In the field of biodiversity conservation, the zoo puts on a fight against the extinction of species based on two pillars. Conservation of species on a quantitative and qualitative level, and education of the public about the depletion of mother earth and its consequences. The latter is often enthusiastically addressed with various educational programs, and information points in the zoos with volunteers as staff. A few decades ago people realised that contribution of zoos to the conservation of species would only be effective if international cooperation between zoos could be established. The 'law of large[r] numbers' plays a significant role on gene, race and species level. Inbreeding, and thus the appearance of hereditary problems, should be avoided as much as possible. Therefore, targeted breeding programs are developed which are centrally managed and directed, and make use of the gene pool of as many zoos as possible. In Europe this has led to the European Endangered Species Programmes (EEP) and the European Stud Books (ESB). The management thereof is appointed, per species, to a specialised zoo and a dedicated employee who bears the responsibility for this task. He or she plays an important role in the maintenance of the collective species population in zoos, and possibly in the wild. For several species programs are developed to try and return "artificially" (ex situ) bred animals to their natural habitat in the wild.
Finally, it is worth mentioning that many reputable zoos fund research and implement it on location for better understanding of the causes and consequences of the degradation of natural habitats on wildlife populations throughout the world, with the ultimate goal to preserve the species.
The World of Zoos
My personal visualised view about the importance of Zoos and the justification for their existence, provided they are member of the World Association of Zoos and Aquariums (WAZA) or one of the regional organisations, such as EAZA and AZA, can be seen here:
There still exist a lot of misconceptions and misunderstandings concerning the aspiration of zoos and the justification for their existence. To achieve a better notion, the plea of Brian Bertram is highly recommended.