Great answer!
I know that some actually have a good enough understanding to initially petition the Lodge for that very reason, but many others only learn it as a part of the instruction that they receive during their journey through the degrees.
I have witnessed a lot of men joining because of their desire to simply belong to a fraternity, and discover the TRUE meaning along the way.
That's why it is difficult to answer the question, what is Freemasonry? It would be like trying to describe a vacation before you took it, for it's a continual journey -- not just a one time trip to Disneyland. :)