The point of view you choose completely depends on what report data you want to see. Each company or affiliate in the Portal can be used to create players, publish players, upload content, create playlists, etc. As for the points of view, the seller company is the company that owns the player, the content owner is the company that owns the content uploaded to the Portal, and the publisher is the one that takes a player with content and publishes that player on a site.
There are also companies that both publish content and create their own content - the so-called combo partners.
For example, you want to see video views at YourBrand company sites. You set 'Publisher' as the point of view, 'Video views' as metrics and cut them with 'Content owner name' dimension. What you will see will be video views per content owner, of all the players that contain YourBrand as a buyerCompany:
But YourBrand is also a content owner. So, if you go to the same report and change the point of view for 'Content owner', you will get the video views for content owned by YourBrand regardless of whether it is published at YourBrand's sites or not:
If YourBrand had not owned any content themselves, this report would have shown up blank with no results.