Coal Coke is created by cooking Coal in a Coke Oven. Even though coal coke is produced slowly in the coke oven (2.5 minutes per piece), it is a highly effective fuel as it essentially produces quadruple the heat content of the Coal required to create it. Coal has a heat content of 1600 (3200 in Railcraft), whereas Coal Coke has a heat content of 6400. This means that Coal Coke will last twice as long in all applications. Its primary use is in the creation of Steel Ingot in a Blast Furnace, but it also makes a good fuel for the Tunnel Bore or for your Furnaces and Generators/Engines (IC2/Buildcraft).
In the production of Coal Coke in a coke oven, a byproduct of it is Creosote Oil which is stored in the coke oven itself. However, if full, the production of coal coke will stop and so it is recommended that the oil is pumped out into a tank for continuous production.
Coal Coke is also able to produce 16000 EU in a generator, 4 times the amount of regular coal (4000 EU per unit).
Coal Coke is also needed to make Railcraft's steam engines (Hobbyist, Commercial and Industrial steam engines) work through a boiler with a solid fueled firebox. Although ordinary coal can be used, it runs out too fast to be of any use in heating up the boiler and producing the necessary steam to power the engines or turbines.
A video showing a use for the Creosote Oil byproduct can be found here.