What's the purpose of taxonomy? What's the definition of a "species"? Should anyone care?
If I wanted to classify the stuff in my house, I could classify the pencil and the broom together because they're both made mostly of wood. Or I could classify the pencil and the pen together because they're both writing implements. Or I could classify the pencil and the oranges together because they're both orange-colored. Etc.
So educate me. What's the point of taxonomy?
Originally taxonomy was exactly like that. We have, for example, ancient 'taxonomies' in scripture such as terrestrial life being classified into cattle, wild beasts and creeping things and plants into herbs, trees and fruit trees. One can see the obvious influence of agriculture in setting these categories.
Until Linnaeus wrote his Systema Naturae, each botanist and zoologist set up his/her personal system of taxonomy. This, of course, meant that it was difficult to decide whether a species described by one person was the same or different from that described by another.
What Linnaeus attempted was a universal system of taxonomy everyone could use with standard names for every species and every species included in the catalogue.
It is important to note that Linnaeus was simply trying to catalogue all species, not create a phylogeny. But his work did generate discussion about which characters count in establishing categories. To use your example: what is more pertinent--the colour or the material? Is it more important that a bat has wings like a bird or that it has fur and mammary glands like a cow? Ditto for whales. Should they be catalogued under fish or under mammals?
Darwin himself made a significant contribution to the science of taxonomy via his study of barnacles. His careful work, especially on their larval development, established their place among the Crustaceans rather than the Molluscs.
Darwin Online: Darwin's Study of the Cirripedia
The significant feature of Linnaean taxonomy was that the classification of plants and animals yielded, in broad outlines, a nested hierarchy. Virtually all known species could be placed in a kingdom, phylum, class and order, with each smaller group belonging to one and only one of the larger groups.
Consider your example again: if we were to look up "pencils" we would find it in two different categories: wood things and orange things. And the other things we found in each category would be very different.
The aim of taxonomy is to provide a unique category for every species based on its natural characteristics. IOW species are not to be assigned to a category arbitrarily or on the basis of superficial traits. Nor should there be any need for cross-referencing which would put a single species into two or more categories depending on which character is used as a marker.
Until the 20th century, the characteristics used were largely morphological. Today, molecular similarity is supplementing/replacing morphology as the basis of taxonomy.
Why does taxonomy play an important role in discussions of evolution?
1. It explains the existence of a nested hierarchy. The nested hierarchy of biological taxonomy was discovered prior to Darwin's time by the research that went into finding the most efficient way to catalogue species. It was noted as part of the orderliness of creation. But no cause other than the creator's will could be assigned to it. Darwin's theory provided a natural explanation. Each category is unique because all members of that category are descendants of the first one of that kind. IOW just as all breeds of dogs are dogs because they are all descendants of the original dog, in nature all the finches of the Galapagos are finches because they are all descendants of the first finches to arrive there. And then one can go on through the higher categories. All species in the same genus belong there because there was once an original species from which they are all descended. And all genera in the same family belong there because there was once an original species from which they all descended. etc. etc. The concept of evolution transformed the nested hierarchy from a mystery into a gigantic but understandable family tree of phylogenetic relationships.
2. Working on the assumption that the nested hierarchy of taxonomy is neither an accident nor a whim of the Creator's will but the outcome of natural process enables scientists to predict possible relationships among species that can be of value in many ways. We have seen, for example, that successful predictions of transitional fossils have been validated by subsequent paleontological discoveries. It can also be used in medical applications both in diagnosis and in testing remedies.
3. Probably the most important test of the theory of evolution was the discovery of the structure of DNA. It was widely understood when we could read genes for the first time, that if evolution is true, then the nested hierarchy based on morphological data should be mirrored by a nested hierarchy based on genetic data. By and large it has been, which is why biologists now speak of the "twin-nested hierarchy".
Of course, there are still anomalies in both systems as well as between them that haven't been resolved yet, and we also have the newly-discovered phenomenon of horizontal gene transfer that cuts across the vertical transfer of genetic material from parent to offspring. But those are questions for present and future research to solve.
