I think the idea is that the hole is aligned so you can sight through it at the Itsukushima floating torii. Unfortunately I couldn't find any pictures of it from the side, so you can't really judge whether Sanae's right...
However you look at it, the bottom half looks like a pair of—My first Miyajimaaaa!♡Stop that.Well then, what to look at first?Kogasa, Kogasa! There's something weird over here!!