Earlier quoted context omitted.
To me it matters a lot. Capitalism only works when the true costs of things are accurately reflected in their prices. If the road maintenance cost of transporting the potatoes is priced into the potatoes, then the potato company might opt to transport the potatoes in a different (cheaper) way. But if the road maintenance costs appear free or near-free to the potato company, they have no incentive to care about it. Al…
The price is included. Potatoes are sold and the proceeds taxed. Those taxes go to pay for roads. If we didn’t have to fix roads, taxes could be lower, resulting in higher profits for the potato farmer.
If road taxes are directly tied to the things causing wear, then every company causing that wear has an incentive to find a transport method that causes less wear (assuming that the additional cost of that transport method is lower than the cost of the road wear - that's why some people argue that using markets is better than using rules; if you've properly priced the market, the market will balance between road and rail based on cost.)
That's why it's important to make sure that all externalities are directly tied into the cost of whatever is creating those externalities.