what for : 表目的,表用途,一般翻译成"用来做什么"
why: 表原因,一般翻译成"为什么",why一般都要 用because来回答
eg. A: This is a knife.
B: What for? (=What is this for?这是用来干啥的)
A: It's used for cutting something.
A: Why do you come here so late?
B: Because my watch is broken.