Billions upon billions of dollars and decades of research have been spent on marketing and advertising because it works.

They don't necessarily want to convince you to buy their product directly. What they want is for you to constantly be reminded of their brand so that when you decide to buy a product, your first thought might be their product. If you're thirsty and you walk into a store, they want you to think of Coke. If you're hungry, they want you to think of McDonald's. Even if you don't really like it, maybe you don't buy their product, but you're thinking of it, so maybe you'll talk about it and remind someone else of it.

Watch data is valuable because it lets them know how to keep your attention so that you'll watch more ads. It tells them which ads to associate with you personally, and which videos to put which ads on for maximum effect.