Some pages use DoubleLines to separate a DocumentMode OpeningStatement, thesis, or pattern at the top of a page and (usually ThreadMode) discussion below. The top part is generally the pages payload -- a short article on the page's title subject along with bibliographic information; while the bottom is meta-data about the page -- discussion, suggested changes, categorization, stories, indirectly related links etc...
One possible part of a WikiPageLayout.
On SeparateThreadsFromContent, someone said "The point is that no one seeking signal needs to read below the double lines."
Reversing that statement would imply that everything below DoubleLines would be noise. As a newcomer, I'd rather see DoubleLines as an separator between stable and volatile content. Eventually as hint for later refactorisations. -- DavidSchmitt
Sometimes it's used to separate different topics on a page. And, as such, it practically begs for the page to be refactored -- and offers some guidance on good break points for the refactoring.