This man is not a publicity seeker per se. He is an artist who seems to be thrilled that what he is doing is getting a lot of acceptance from the public. He seems to want to do good and actually be happy, and Oprah is certainly a role model for that so this venue is a fine one.
Exposure is key to generating sales in the music industry. As well as consistent positive reviews for the show, it has a proven track record of high viewing figures. The target audience of the Oprah prime show not only allows "Happy" to reach the younger audience members but also some of the population that without this exposure would not normally have heard of, or purchase this music. It is an excellent forum for an artist to show not only their talent but also their personalities.
Pharrell Williams probably made one of the best decisions of his career by discussing his hit song on Oprah's Prime TV. Not only is Oprah known but she has a huge amount of influence in the African American community as well as many other viewers. Her following could potentially be his following.
I think Oprah Prime was a decent choice, but it cuts many viewers off since a lot of people are cutting the cord. Williams' song "Happy" is a great piece of art that the American public needs. It makes people feel good, even though the times are hard. Oprah Prime is not the only outlet for this music as it's being played over and over again on several different platforms. This is a song that essentially does not need promotion.