What is the purpose of a purpose, what role does a purpose serve in the grander scheme of things: why direct?

A purpose is a use. There is not "grander scheme of things". To ask what is the "purpose of a purpose" is the same as asking what is the "use of a use". Use is normally not a self-referential concept and to treat it as one would probably lead you to a paradox.

"Purpose" implies intent, which involves the ability to preconceive a goal to act towards, which is only an ability of agency, which is itself only the result of a brain organ. "Arriving at our purpose" suggests reaching a preconceived goal of the application of one's life. One would have to establish the existence of some kind of grand brain, like a deity, to suggest some kind of grand "purpose". Purpose is not the right word when you are talking about the causal relationships between unconscious natural phenomena.