Why: I feel like this would be one of the easier features to implement and it would open up a whole lot of possibilities.
Example: In my case, I am building a website for writers to promote and review each others work and video reviews of literature is a pretty popular thing in the YouTube community. So I want to use this feature to be able to give my users the ability to display their YouTube video review of a literary text as a comment in the comment section on my site. That way I get some cross platform promotion, my site gets an entry point into the YouTube community which is quite large, and the users get a more personal way of facing each other.