Here is one reasoning I have read for the use of open em dashes (with spaces) instead of closed ones (without spaces). Some fonts have a very narrow em dash. The em dash was supposed to be used closed because it is was large enough to create a visual break. Very narrow em dashes fail in this case. If you are merely providing text without marrying it to a specific font leaving the font choice to the reader (like some e-book viewers do) then you might decide to add spaces around the em dashes in fear of the very narrow ones.
Another reasoning sort of inspired by "The Elements of Typographic Style" (and rather exaggerated in my opinion) claims that closed em dashes without spaces fit with the aesthetics of a more vintage typographic style (verbatim: "Like the oversized space between sentences, it belongs to the padded and corseted aesthetic of Victorian typography.") while the open ones have a modern feel to them.
Something to mention, sometimes the "house style" indicates that em dashes should be set open but using thin spaces (such as
) which depending on the context can give the illusion that they are closed.