Lacking a more definitive source, I'm working from the metaplot article on Wikipedia, which says, in part, that
The metaplot is the overarching storyline that binds together events in the official continuity of a published role-playing game campaign setting. Major official story events that change the world, or simply move important non-player characters from one place to another, are part of the metaplot for a game.… Because of [some unpopular] events…, many gaming groups choose to ignore the metaplot for a game entirely.
Metaplot information is usually included within gaming products such as rule books and modules as they are released. Major events in the metaplot are often used to explain changes in the rules in between versions of the games….
(I've used the Wikipedia definition here because it was concise, if limited in its application. The TV Tropes's definition is better but more loquacious.)
What I'm struggling with is seeing any upside to a metaplot. By that I mean it seems like a role-playing game's setting will typically be defined by the game's initial release, and if that setting is later altered by the metaplot, future expansions must either accommodate and perhaps (because of previously established expectations) advance that metaplot to satisfy those who are familiar with the now-changed setting and potentially alienate newcomers or ignore the metaplot, leaving fans of the metaplot bereft but making the game easier to grok for newcomers. This sounds to me like a metaplot is a lose-lose proposition.
In short, then, besides disguising mechanical changes behind a narrative façade, why do roleplaying games have metaplots? Has an author or publisher revealed his or its motivation for the inclusion of a metaplot?
I am looking for a definitive answer like As the author of a role-playing game, I opted to include with my game a metaplot because and As a roleplaying game publisher, I prefer my company publish roleplaying games with a metaplot because and similar answers derived from insider knowledge rather than speculation. However, such insider knowledge can be shared by those far distant from how the sausage is made or by those outsiders who are otherwise somehow extremely well informed.
Note: I was considering work once again on my homebrew science-fantasy heartbreaker and was considering a metaplot, but then I realized I had never heard anyone say Wow, the awesome metaplot really ties the setting together! or, pretty much, anything good about a game's metaplot. However, the metaplot concept is so ingrained within the roleplaying game medium—so much so that I was considering it for my own work—, yet, given a metaplot's controversial nature, I don't know why.