1) Presence. Right now Google often doesn't know WHO is watching YouTube, whether it be the logged in user or someone else in the house. If they know you're watching on a desktop or TV but your activity shows you're walking around its likely you're not actively watching or someone else is. They could hold off on showing an ad or not target it directly to you until they think you're there to watch it.
2) Involvement. Wasn't exactly sure what to call this but what I mean by this is they can measure your heart rate to see how you react to various things you're looking at now. There might already be studies on how best to control peoples heart rates when buying something or keep them watching longer but if not they'll be able to do a ton of machine learning to test this out. This can help determine when to run ads on YouTube videos to keep you from clicking off or when the optimal time would be to show you an ad to cause you to buy something.