Why an Agnostic Church at all?

Because there is a "religious tendency" in the human race, it was developed as a basic survival need in our distant evolutive past as it is explained in the book of Matthew Alper "The God part of the brain"
He demonstrates that we all have a "spiritual/religious instinct" more or less genetically hardwired in our brains

My advice in this matter is to find a way to rationally exercise our "mystic muscle", to use our "religious instinct"

How doing something apparently contradictory like "believing" in Gods without renouncing to our reason that is our first and only weapon to survive in the reality?
Well... the answer is easy: "Believing" in Gods without ever forgetting that we invented them, this means in practice that we can pray, worship images or icons, having amulets, etc. but always keep in mind that there is nothing "really real" out there, nothing super-natural, nothing outside the inalterable physical laws that govern the Universe. Our Gods are "alive" only inside our minds.

Why doing something like this??

Because it is healthy to exercise every instinctive part of oneself in some way compatible with one's own rational values and moral principles. Sex, curiosity, fighting, parenthood, to name a few are other instincts that should also be exercised in order to be more in harmony with what we really are.
As Aristotle taught us: A is A no matter how strong we would like to be otherwise.