The Shale Plant provides the and of the Coal Plant it replaces without requiring Coal . This makes it good if you can't find Coal, but still have the extra to handle the that comes with Coal Plants (which can be provided late-game by the Environmentalism civic). In addition, it gives +10% in any city that it is built in, which makes it even more helpful for a high city.
In the early half of the twentieth century, Japan was fast becoming one of the most industrialized nations in the world. As such, Japan needed access to fossil fuels to run her factories, energy plants, and vehicles, especially her growing navy. Much of Japan's colonization/conquest efforts during that period were designed to gain control over areas rich in such resources. Conflict between Japan and the United States - another growing Pacific industrial power - over these resources was one of the root causes of the Second World War (or the Pacific theatre portion of that war, anyway).
Shale is a compound formed of decayed and compacted organic material much like other fossil fuels. However, shale requires a good deal of preparation before it becomes an adequate fuel, making it much more expensive than petroleum. However, lacking easy access to high-quality coal and petroleum during the pre-war period, the Japanese constructed a number of plants designed to create fuel from shale. The shale plant in occupied Manchuria was capable of producing as much as 200,000 tons of shale oil per year. Such plants kept the Japanese navy afloat during its long struggle with the larger and resource-rich United States.