Media agencies/advertisers can make advanced purchases based on data and environment on 1000’s of sites without having to contact them directly. There are several ways of buying programmatically, but most often the publisher or a representative sets up a private deal that gives the advertiser a guarantee of what they are buying.
For a publisher, this means that 1000’s of advertisers have access to bid on your ad slots instead of going through a manual sales process. In addition this means that advertisers can be dynamic in their setup since it hardly requires any administration from the publisher side. Programmatic creates completely new conditions and thereby increased revenue potential.