Its general purpose is to arrange plants in such a way as to provide an information retrieval system, i.e. the name and rank or position in an ordered hierarchy tells us something about the plant.
Taxonomy is probably the oldest science - plants classified as poisonous vs. edible.
Classification is a natural occupation of humans and one of the most necessary of pastimes. It is essential to our daily lives and is started almost as soon as we're born. Taxonomy is a very dynamic and synthetic science and is basic to all other sciences, yet at the same time dependent upon them, i.e. the information used by taxonomists is gathered using information from other disciplines such as genetics, ecology, morphology, anatomy, physiology, etc. On the other hand, all other sciences are dependent on taxonomy, e.g. other disciplines must have the correct name of organisms they are working on, e.g. cancer drugs or antibiotics. To be able to collect and prepare herbarium specimens.