Your audience only sees content in public collections and private collections to which they are subscribed.  So how can you make your channel interesting to those who are not yet subscribed? How can you convince them to subscribe to collections with fee-based subscription? The answer is to promote "Free samples" which are visible to all.

Your questions answered

I can't see the promote button shown here

The promote button is only available on collections with the pay-to-view service, so you won't see this button for videos in fully public or private collections.

It is also necessary to be in edit mode in the Content Manager application to access the promote button.