Water rolls off the roof tiles* into a gutter and down into this pipe and then up again into the main body of the construction.
On a separate note, V recently remarked to me that she now regrets not hiring Richard Rogers to design our house, as she's lost track of where many of the pipes are. (And it's not that we don't have a proper set of blueprints.)
*Laminas in fact. Actual tejas were apparently deemed a luxury extra by this developer.