Adventuring characters gain experience through the wealth they extract from the underworld. As detailed in The First Fantasy Campaign by Dave Arneson, adventurers in the initial version of what was to become D&D were required to spend their plundered gold pursuing certain motivations in order to gain experience from it. Gold allows experienced adventurers to bring order to the wilderness on the surface through the construction of strongholds. The forces of Law desire the plundering of gold from the clutches of Chaos in the underworld that they might spread the will of man across the land.My imagination went to a slightly different place with this idea than Sham took it. He went in a metagame direction (XP), while my thinking was more economic. Both approaches are slightly different aspects of the same thing, and lead to similar outcomes.
The more gold in man's economy, the more the domain of Law expands, thereby encroaching on the domain of monsters. It's habitat destruction.
Most monsters of low and medium intelligence have no use for gold. Their "societies" are anarchies where the strong dominate the weak. They have little use for abstract currency, since their economy is based on barter—barter of goods, or the barter of favors and promises.
However, some intelligent monsters recognize the dangers posed to their habitat by man's expansionism. These monsters, particularly dragons, hope to stymy civilization by removing currency from the human economy (i.e.—hoarding treasure). They do so directly and by employing lesser agents, such as orcs, goblins, and kobolds. Goblins have no interest in gold, but they desperately want the favor and protection of a dragon or his mid-level ogre operative.
Low level adventurers are just as much cogs in the System of Capital as goblins. This is even more evident if you use the carousing rules in your game—characters acts as hoses, syphoning gold from the dungeon to the economy. Even neutral, primarily self-interested adventurers are therefore agents of Law.
Law versus Chaos is all about the flow of capital. Dragons are economic terrorists, who want the flow running away from Men.