The concept of the battlecruiser was borne from a great many factors. Consider the situation of the British Empire in, say, 1900. They have a huge empire, spanning the globe, that they protect and maintain through their unquestioned naval superiority.

But, things change constantly. New concepts - the torpedo boat, for example, threaten British supremacy. Most of the Royal Navy was built around pre-dreadnoughts with a top speed under 20 knots. They were plodding monsters that can easily be seen falling prey to a swarm of cheaply built ships equipped with torpedoes that can overwhelm them with numbers.

The concept of raiding cruisers was also not totally new at this point: ships that were relatively fast but able to disrupt sea lanes have existed for a long time. The British needed to be able to deal with these ships.

Additionally, Jackie Fisher was expressly sought for the position of First Sea Lord because he recognized that the economic implications of naval shipbuilding were actually an issue to wrestle with.

How then, do you build a fleet that is a) able to span the globe (the German navy could barely venture out from the North Sea), b) survive the torpedo boat threat, c) deal with the cruiser-raider threat, while d) providing the "prestige" and "power" of the battle line, and e) do this at a reasonable cost?

Jackie Fisher thought the answer to this was the battlecruiser, and it's not really that difficult to see why he would think that. Battlecruisers were swift enough to outrun torpedo boats and the like, could deal with fast raiders with ease, had the size necessary for stores to patrol the globe, and had the power and appearance of the battleship, wrapped into a nice economical package.

Of course, the British didn't totally buy in and they built both battlecruisers and Dreadnoughts. :)