Before oil companies could make heaps of moolah, they had to create a need for gasoline. This required convincing early auto owners, who primarily drove short distances in urban settings, to take longer drives. But how, and where? At the beginning of the century, few roads were suited to car traffic, and they were not marked on maps. So, oil companies began producing maps -- with their names or logos, naturally -- to be distributed gratis at filling stations.
This sounds simple now, but involved monumental effort. Not only was it a marketing campaign that created an entirely new motoring culture, but the oil companies not only mapped and marked but in many cases created the roads themselves. State and especially federal government involvement came much later, and often followed routes that had been laid out by the oil companies.
Oil company maps were also highly influential in developing touring behavior (i.e. driving around seeing things rather than getting to a destination) and encouraging women to drive.