What do they get out of this? Well, with a increase in the views and subscribers, the Youtube channel owner gets income and he could share that income with his friend. 
How do we validate that the winner is not a friend or family member? Well, it's basically impossible and that is the main problem I have with these puzzles.
It is a realistic possibility that people are coming up with new ideas to make money with all these puzzle and rigged prizes but we have to give him the possibility of doubt until it is proven and there is nothing you can do about it, you can make a decision whether to participate in his game or not and people with time might spend their time chasing a puzzle. How do we validate that the winner is not a friend or family member? Well, it's basically impossible and that is the main problem I have with these puzzles.
@OP hopefully the winner will be an old user from this forum rather than a newbie account claiming the prize.