I think the terminology is misleading and it should've been replaced with something more explanatory like "videos from YouTube partners". And just because the videos are from YouTube partners, it doesn't mean they're more relevant than the "organic" results. YouTube's parent company doesn't have a special section in the search results for websites from Google's content network.
Besides earning revenue from advertising and being promoted on every YouTube page, partners have many other privileges: they can upload custom thumbnails, create branded channels and offer downloadable videos.