The other layers act as a set of progressively smaller pored filters, they'll remove larger particulates and keep it from gunking up the finer stuff even though the heavy lifting is done by the charcoal like you say. Also I hear if you use limestone it makes the water sweet, supposedly this is the "secret" to Jack Daniels, although personally am not a big fan of JD.

Isn’t it odd that as soon as we talk about something, it shows up. I wonder if this guy saw our comments.

Or could be the Jungian collective unconscious at work.