Cladistics is useful for creating systems of classification.
Cladistics is now the most commonly used method to classify organisms. Why do we need to classify organisms? Well, consider the bewildering variety of organisms that have ever lived on Earth, from jellyfish to bacteria -- that's what paleontologists do for a living. How is it possible that paleontologists, let alone other biologists, are able to communicate their ideas about such a diverse topic as the history of life? Well, it's obvious that a system of classification is needed. That is, we need words like beetle or conifer so that we can talk about many organisms at one time. In fact, the history of formal classification schemes in biology is long, dating from the 1700s, well before Darwin proposed his theory of natural selection. Today, cladistics is the method of choice for classifying life because it recognizes and employs evolutionary theory.
Cladistics predicts the properties of organisms. As with any other system in science, a model is most useful when it not only describes what has been observed, but when it predicts that which has not yet been observed. Cladistics produces hypotheses about the relationships of organisms in a way that, unlike other systems, predicts properties of the organisms. This can be especially important in cases when particular genes or biological compounds are being sought. Such genes and compounds are being sought all the time by companies interested in improving crop yield or disease resistance, and in the search for medicines. Only an hypothesis based on evolutionary theory, such as cladistic hypotheses, can be used for these endeavours.
Cladistics helps to elucidate mechanisms of evolution. Unlike previous systems of analyzing relationships, cladistics is explicitly evolutionary. Because of this it is possible to examine the way in which characters change within groups over time -- the direction in which characters change, and the relative frequency with which they change. It is also possible to compare the descendants of a single ancestor to look at patterns of origin and extinction in these groups, or to look at relative size and diversity of the groups. Perhaps the most important feature of cladistic is its use in testing long-standing hypotheses about adaptation. For many years, since even before Darwin, it has been popular to tell "stories" about how certain traits of organisms came to be. With cladistics, it is possible to determine whether these stories have merit, or whether they should be abandoned in favor of a competing hypothesis. For instance, it was long said that the orb-weaving spiders, with their intricate and orderly webs, had evolved from spiders with cobweb-like webs. The cladistic analysis of these spiders showed that, in fact, orb-weaving was the primitive state, and that cobweb-weaving had evolved from spiders with more orderly webs. This situation has been repeated in many groups with many traits, including studies of parasitism, geographic distribution, and pollination.