The division or classification of anything is done for ease of information storage, retrieval, transfer and propagation. It is much easier to refer to a smaller class of anything (say sedans, pick-ups, etc.), rather than entire body of those substances (say, automobiles).
Classification simply means grouping things together based on some properties or parameters. Substances can be classified as elements (classified as metals, non-metals, noble gases, etc.), compounds (composed of two or more elements) and mixtures (which can be further classified as homogeneous and heterogeneous mixtures).
It is simpler to store (or arrange) the information or substances according to some classification system so that its retrieval is simply a matter of searching from within that particular class of substances. Easy finding of information or substances enables people (and researchers) to use them and search for further information. Hence, classification is preferred.
Hope this helps.