The Harbor of Hearts' campaign against the Alchemist was enabled in large part by easy access to cross-country teleportation networks, facilitated by the powerful spellcasters gathered among the Marked and the resources and knowledge obtained over their guildlord Adrianna's decades of adventuring. But before that networked was established, a guild was established to provide one basic service: moving goods from point A to point B. And though the Courier Knights' services weren't often in demand among the Marked, they were of great value to an important secondary demographic: literally everyone else.
After the Worldweep fell, Golarion entered a period of rapid redevelopment. And in Avistan, most of that development would have been impossible without the supply lines established by the Courier Knights. Their network rapidly expanded beyond Moonlit Village and the nearby town of Snowmelt, and with each new delivery outpost established, new couriers were added to their ranks, allowing them to expand their services to more and more settlements further and further away from their original headquarters. All of the knowledge and resources that spread from Moonlit Village (soon Moonlit City) to the rest of Avistan traveled along the Courier Knights' delivery routes, and the Knights' dedication to rebuilding Golarion ensured even the most remote villages and outposts would have access to their services. Eventually their network spread even into Garund and Casmaron, and though they haven't yet been able to establish regular routes to Arcadia and Tian Xia, some people believe it's only a matter of time.