The WAN as initially conceived was about one simple job: the WAN was the network that “connects my sites to each other.” That is, the network connecting users in corporate sites to corporate IT resources in other corporate sites or perhaps colocation facilities. It was all inside-to-inside traffic.
Over the past decade so much has changed that, just before COVID-19 work-from-home mandates took hold, only about 37% of a typical WAN’s traffic was still inside-to-inside, according to Nemertes’ “Next Generation Networks Research Study 2020-2021”. The rest touched the outside world, either originating there as with remote work against data-center systems or terminating there as with SaaS use from a company site or both as with VPNing into the network only to head back out to a SaaS app.