OK so I work for Microsoft but hey it’s worth a try …consider the Edge Transport role server as a replacement to other services in the perimeter.
You should only need deploy a very small number of Edge Transport servers in your environment and you may have the opportunity to replace a number of other mail relay hosts that currently take care of your email when it’s on its way to or from the internet. 3rd party solutions are used for message hygiene, encryption and other services such as address rewriting and adding disclaimers. It might be the case that you can save on 3rd party software licenses by deploying Edge Transport servers only. Management of your solution is simplified and supportability costs reduced.
This whitepaper is definitely worth a read if you want to know more about how Microsoft deploy their Edge Transport role servers: Exchange Edge Transport Servers at Microsoft