"What are you for" could carry two meanings. The first is "What are you (good) for?". Here we are asking what the purpose of something is. If I am in a toolshed and I see something confusing (like a spanner), I might ask the spanner "What are you for?". Hopefully, the spanner would not talk back to me. The second is "What (views) are you for?". Here we are asking someone what their opinion is on something. I might say "I am for freedom of speech" as a way of saying that I support it. The sentence "What are you on the lookout for?" has a completely different meaning - it is the same as "What are you looking for?"

'What are you for?' is not a usual expression at all. It's not something that we would ever normally say. Are you perhaps thinking of 'What are you AFTER?' ? This is an informal question meaning 'What are you looking for?' or 'What are you trying to find?'