Rocks are a type on terrain only found in open land, where nothing can be built at first, making them useless early in the game. From the 
Industrial Age onwards however, some resources become visible on the map, and most Rocks may end up containing a resource and becoming a valuable asset, most importantly of them all, Oil. Although the presence of Rocks doesn't guarantee the presence of Oil, it does exclusively appear on Rocks, the only other sources of Oil found in Industrial age Rare Resources, which also tend to sit upon Rocks but provide far less, and Oil patches in the Ocean.
In the Scenario Editor, creating Rocks is required before creating an oil patch.