From Longman Dictionary of Contemporary Englishwhat (...) for?what (...) for?a) REASONused to ask why someone does something ‘She’s decided to work part-time.’ ‘What for?’ What did you do that for? b) REASONused to ask what purpose or use something has What’s this gadget for? → what
Examples from the Corpus
what (...) for?• What's this tool for? 