If you prime before you do the pre-flight were exactly do you think that the fuel goes wile you are walking around checking the aircraft?
Of course it flows down through the induction tubes to the carb, due to gravity. Along the way some of it also becomes vapor. Vapor is good when it comes to starting and running the engine. If you pump the throttle it all ends up in the airbox, unless the engine is spinning round enough to pull it up.
That's what I think.
-- IFMU