why...有两层意思：
1、Why...= For what reason...为什么?= 什么原因?针对 because.提问.
I came late because I missed the first bus.
Why did you come late?
2.Why...= For what (purpose) ...= What...for?为了什么（目的）针对 (in order) to do sth提问：
I came to ask you for help.
Why did you come?
= For what (purpose) did you come?
= What did you come for?
你来有什么目的?你是来干嘛的?