a phrase question

did you mean how come (with an 'e')?

I don't think they are equivalent. Both are colloquial phrases. Where I live, (Britain) "What for?" means, roughly, "why?", or "What is the reason (for what you just said)?" whereas "How come" is American slang and means roughly, "How did that happen?", maybe in the US they are used interchangeably.

"How come" is actually the short form of the quite polite "How has this come to be that ..." and it often means "Why".

A: I'm gonna go to the store?

B: How come?/What [are you going there] for?/Why [are you going there]?

A: To get some beer. Ya want anythin'?

B: Yeah, get me a coupla bags of potato chips.

Thank you very much

Americans frequently use the locution "how come" to mean why. I suggest that that is the most common meaning of how come.