What is Why?
Why is 1. (asking a question) for what reason Why did he have to phone me in the middle of the TV film? I asked the ticket collector why the train was late. 2. agreeing with a suggestion Would you like some lunch – Why not? 3. giving a reason She told me why she couldn’t go to the party. 4. showing something else is preferred Why go by plane to Paris when you can take the train? Why don’t we go for a picnic as it’s fine? (NOTE: After why used to ask a question, the (verb) is put before the subject: Why is the sky blue? but I don’t know why the sky is blue.) (interj) showing surprise Why, if it isn’t old Mr Smith!
source: Easier English, Student Dictionary Upper Intermediate Level