Because it’s giving us information about how we’re feeling and what we need to do about those feelings. When I’m looking at your facial expressions, I get signals—like either you’re interested or you’re bored. Either you want me to keep going or not. And that’s data for me. Because if I notice a shift in the way you’re engaging with me while I’m speaking, I could either use that information, or I could keep going and going and going, and then lose my audience.