The two capital-ship types had different strategic jobs. A battleship was intended to establish command of the sea through fleet battle, while a battlecruiser used speed to interfere with enemy trade and protect friendly commerce. Trying to merge those roles created many of the design compromises seen across the dreadnought era. HMS Dreadnought's 21-knot speed, faster than most merchant ships and many older battleships, also began to blur the inherited separation between slow line-of-battle ships and cruisers.
Chapter · 4:00-5:35
Battleships and battlecruisers as different sea-power tools
The opening distinction is between battleships built to win fleet action and battlecruisers built to control trade routes and hunt enemy cruisers.