It is important because if there no diversity everything would be the same. When everything is the same there is no change and disease can rise. When everything is the same there is no chance of finding something that can help to cure sickness etc.. because everything has the same genetic and so all go down together having eh same immunity flaw.
Without diversity everything would look and appear the same as well. There would be no variation. Some people might regard this as a positive thing, and build up an imperialist culture; however more often then not it leads to insanity and destruction of culture when 'all that is the same' is discovered as being 'low of worth due to the fact that it is the same', or 'able to be conquered' due to the fact that everyone is the same.
Without diversity there would be no cycle of nature. The world of nature would be flat and not exist in flourish. There would be no large or small chain, and this would lead to disease infestation and also starvation.