Negative Audiences are groups of people that you want to exclude from advertising to on the Google Display Network. Commonly excluded audiences, or negative audiences, include converted visitors or visitors that bought a product on your e-commerce store. Negative audiences can be added to your display network campaign through campaign exclusions. Tracking converted visitors in Google Analytics and Adwords allows you to better target your advertising. Just like you have keywords and negative keywords, you have audiences and negative audiences.
You could annoy the customers that have bought from you, waste advertising spend, or both. But more importantly, negative audiences allow you to better manage your funnel and include and exclude visitors based on the actions they take on your website.