With legacy code, and things like this that don't make sense superficially, think about other elements that might have been there before that needed disambiguation, but were changed to what you see.

Here's an overly dramatic explanation of a different kind. Just imagine that originally there was an empty hash constructor, with a leading plus: +{}. Sometime later it was changed to an empty array, +[]. Since the expression compiled with the plus, and didn't change the result, it was overlooked by the author and left to furrow future brows.

(I must admit Poe's Dupin is my favorite detective, even above Sherlock, precisely for this mind-reading ability, fantastic as it must be.)