Because there are some fish that can only survive in Coral reefs :/ And im sure adapting right away to the open ocean is not going to be good. On top of that we destroy a lot of Coral reefs when we have oil spills from huge boats. And preserving our nature is always a good thing because helping coral reefs could mean saving lots of coral and tropical fish and keeping our earth as beautiful as it is:)
hope this is helpful
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