There were many problems that came with living in an apocalyptic wasteland. The scarcity of resources being a major headache for all who wished to survive. And though Gravestone had for the most part solved the issues regarding food, water, and shelter. There was still one major resource that they were lacking.
It's hard to think of a world without modern transportation. Whether that be automobiles, trains, or airplanes. But for the vast majority of human history, these things did not exist. And in this post apocalyptic world was worth its weight in gold.
In fact, it was worth a lot more than that. Sure, there were diesel engines which could regularly make use of bio-diesel. But few had the means to produce such a life saving substance. Few outside of Gravestone, that is.