purpose has 11 distinct synonyms spread across 5 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
what something is used for
“the function of an auger is to bore holes”
the quality of being determined to do or achieve something; firmness of purpose
“his determination showed in his every movement”
Synonyms, antonyms and the broader and narrower relations for purpose come from Princeton University’s WordNet. Full sources and licences.