Keeping fish, especially rare and endangered species gives these species a monetary value. If something has a monetary value then it is valuable to the economy from which it comes and so is worth protecting. Something that is worth protecting is more likely to last in the wild if managed properly. Not only are the animals protected but also the environment from which they came.
On a more personal level I find them relaxing, attractive, educational and exotic yet unobtrusive, clean, quiet and relatively undemanding. Try keeping a dog with just 2 minutes a day care and a weekly hour or two of maintenance. Not going to happen! Well, not without the law getting involved anyway.
WK