A sandbox has to provide the resources and tools for DM and player for the campaign to go the direction everyone is willing to let it go. To paraphrase a popular saying: If your campaign is full of hammers the players are going to treat problems like nails. The tools and resources a DM puts into their campaign will impact where it goes and how it plays out and what goals will likely be pursued.
People will tell you OSR games and campaigns are about exploration. There has to be something worth exploring to make it worth it. In real life people explored to gain access to wealth, that wealth could be gold, iron, timber, or furs. Routes to reach that wealth quicker and safer are also a prime reason to explore. The location, depths, and widths of rivers and bays were of prime importance because they gave access to other resources. The places to go had something to offer the traveler and motivated the explorer beyond tourism.
Those who build their own castles to rule from (or protect other resources) need the means to build a castle. To construct a castle one needs access to quality stone, timber, man power, and skilled master artisans that can make use of the man-power and materials. Sure there’s lot’s of stone, but even that is a limited resource it and timber must be cut and hauled to the construction site which reveals the most limited resource beyond skill: man power. Gaining access to that manpower and being able to keep it at work is an adventure unto itself.
What of food? Where does that come from, how does it travel, who grows it? Food is an essential resource and the production of food is a foundation of the entire feudal system that many a pseudo-medieval setting would depend upon. It’s a limiting factor in everything the powerful wish to achieve and the ultimate source of their power. One need not delve into the mircoeconomic of an agrarian society but there does has to be places where the food is grown and routes by which it travels.
Where do the steeds come from? Who makes weapons and armor? Where do good actually come from. When your players only need to supply a handful of people the market can serve it’s needs but when armies and nations need to be fed these issues become far more important.
Transporting needed resources requires skilled agents to transport the goods with vehicles capable of carrying those goods along secure routes. An empire can rise or fall on the delivery of grain, timber to build ships or wagons, and personnel that can tie the pieces together.
Treasure is a resource of course. Treasure is a means to get others to labor to deliver resources to the treasure holder. Treasure is portable access to power and resources. A chest of gold does not make a man comfortable, but a chest of gold can buy an estate and keep the cellars full of supplies.