Many companies have good reasons to keep their messaging infrastructure on-premises. Exchange 2016 is currently the second latest version of Exchange Server for on-premises deployments. Common reasons to upgrade Exchange include adding new functionality, such as high availability, moving from an unreliable or insecure system, and moving to a version that Microsoft still supports.
ENow Software's Exchange blog built by Microsoft MVPs for IT/Sys Admins.
Jeff Guillet MVP, MCSM
Microsoft Certified Master & MVP: Exchange Server MCITP 2012+Messaging+Lync+Virtualization | MCSE+Messaging | CISSP
Microsoft first introduced the Edge Transport role as one of the five Exchange roles in Exchange 2007 and offered it again in Exchange 2010. The purpose of the Edge server role is to provide a solution for customers who require inbound SMTP connections to terminate in the perimeter network (DMZ), rather than in the internal network. Since most inbound SMTP connections are unauthenticated, some security departments are uneasy at allowing these connections directly to internal resources (your Exchange servers). Edge transport servers allow these customers to deploy Exchange without having to buy an SMTP gateway appliance.