An introductory expletive.
Because of which; for which.
As a relative, the reason or cause for which.
For what cause, purpose, or reason.
Why are you in it? – The Pioneers by Katharine Susannah Prichard
But why was it there? – Journeys Through Bookland V2 by Charles H. Sylvester
And why do you go? – The Necklace of Princess Fiorimonde and Other Stories by Mary De Morgan