Principle of commitment & consistency -- When people commit to something, they're far more likely to purchase from you. That's why getting them to agree to something like a free + shipping offer or by agreeing with something you've said in some way. This is a powerful principle in sales and if you pay attention to some of the best marketers in the world, you'll notice that they work fervently to get your commitment to something, even if it's very small in the beginning.
While there are several platforms for doing this, clearly YouTube is the most popular for doing this. However, video marketing is also a great form of both content marketing and SEO on its own. It can help to provide visibility for several different ventures, and if the video is valuable enough in its message and content, it will be shared and liked by droves, pushing up the authority of that video through the roof.