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"Species" is not a clean concept in a world with viruses, clines, and ring species.
More precisely, "species" is a map marker made by someone who likes discrete, mostly tree-like maps (legacy of Aristotle?)
"Species" is one rung on the phylogenetic ladder. Whether a given edge case should be classified as a species or as a subspecies can be debated, but in practical terms it is useful to have a tree-like map, because it allows you to assess the phylogenetic distance between two groups.
Also, compared to the range from class to genus, "species" is relatively clear-cut.