Hmmm, well, the main villain is basically the most cliché BBEG you can imagine, and his "motivation" (ruling the world) is pretty much "I exist so the PCs and I can have a big Hollywood-style final tussle". He even said once that he gotta save the A-material for the PCs (when he Ghostforms through the Azurite watch tower). So he's there as the story's stereotypical BBEG. Period.
The OotSverse is first and foremost a campaign world for our six PCs. Sure, Rich's an awesome story-teller, and some characters are extremely well developed, but it remains that, in many cases, a character being there for no other reason than the fact the campaign/story calls for them to be there -- mimicking the typical roleplaying story -- is something perfectly normal in the OotS world.