How is knowing a species' evolutionary history important in its classification?

Classification systems pre-date the theory of evolution by quite a few years. Back then they were just grouping organisms by structural similarities. When the theory of evolution became well developed it soon became apparent that classification systems closely resembled evolutionary descent as closely related species are similar in many ways. One of the deceptions of the situation however is the case of convergent evolution where widely different creature may look quite similar. in these cases it helps to know the evolutionary history as well as appearance.