Why were castles built?

Defense. The history of Britain and Europe was a constant stae of war and skirmishes for the last several thousand years. The easiest way to keep your livestock (and serfs) yours was to put them behind large walls when your neighbours decided to attack.

Catsles developed from Hill Forts, to Wooden Motte and Bailey catsles, to the stone medieval castles you mention. A brief history of castle building shows their development (http://www.ecastles.co.uk/history.html) As well as defense, they served an offensive purpose, allowing troop garrisons to be maintained for raids and attacks on rivals. Further they were status symbols, showing the power and wealth of the King or Baron they belonged to.

This was because they were not only impressive structures (often covered with white plaster, rather than the stone you see now) but also expensive to maintain. This has been asked before - see