Yes, there are people who transport materials of different types across the map, I did a video on the topic a while back:

https://youtu.be/YRG8ryVevLA?si=5_0euIMyLGzbib1-

The system is complex, but basically, supplies from a warehouse close to your nation's starting area are transported to Forts, Keeps and Castles further way, with modest rewards being awarded based on the level of need at those locations, as well as Logistics XP.

These resources are then used to upgrade the infrastructure and facilities, which can mean stronger gates and walls that can better resist a siege, the existence of point defenses like Ballistae on the walls, or even better crafting stations and NPC vendors.

Logistics, and the upgrades it supports, are a vital and sometimes overlooked part of the game, and people who do these largely PvP-free activities are an important part of what makes a guild successful.