noun
BAD : The purpose why I have come here is to improve my English.
GOOD : The reason why I have come here is to improve my English.
BAD : The purpose of staying in the United States is to complete my PhD.
GOOD : My reason for staying in the United States is to complete my PhD.
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purpose = what you hope to achieve by doing something; aim: 'The main purpose of the trip is to see Helen's parents.' 'Their purpose in coming here is to promote Australian universities.'
reason = the thing that causes someone to do something: 'The reason why she left him was probably to do with his drinking problem.' 'Did they give you a reason for rejecting your application?'