If you extend this thinking far enough, they have a lot to gain. It's an extra dimension that Google can use to further refine their ad inventory against your interests. The theory is that you will see more relevant ads and Google will earn more because of it. i.e. if you like this video, and someone else liked this video, and they clicked on that ad, then maybe you'll click on that ad.
Perhaps on its own this single dimension doesn't add all that much value, but in aggregate and with a mass number of participants, it should.