First, they kill a lot of monsters.
Many entrepreneurial merchants may happily take advantage of warriors killing monsters for trade, fur, food, etc. Even then, clearing the path will open up trade and tourist routes, making the world more safe to traverse. Traffic should increase as a result. More spending on infrastructure should speed this up.
Second, their accumulated gil will circulate back into the economy. Your heroes will be rich. If there is an equivalent of the stock market, some of your characters might speculate with their money. They may even open up businesses and shops themselves, or give out loans to other would-be entrepreneurs.
Clearing up trade routes and circulating capital into the economy is a good thing, in the short term. But unfortunately, this may have negative effects in the long run.
Adventurers will sell monster carcasses for food and leather. A ton of entrepreneurs will also roam the land in hopes of making it big and striking it rich.
Mom and pop shops might expand, and hire new workers in the process to mass produce goods. These shops will turn into full-blown companies, and the risk that they will exploit their workers is also possible.
But then again, it’s for the good of society right? We still haven’t decided whether or not exploitation is bad in-itself. Our heroes routinely exploit their opponent’s weaknesses to gain an advantage.