In 1984, why does Winston have a diary?

In George Orwell's dystopian classic 1984, protagonist Winston Smith purchases a diary and begins recording his secret thoughts therein. As a literary convention, Winston's diary entries help move the story along, but for Winston, the diary entries are far more serious; they are thoughtcrime. Winston's first major thoughtcrime is repeatedly writing "DOWN WITH BIG BROTHER" in the diary. This action constitutes his first act of rebellion against Big Brother and the Party, and it is not his last. The simple action of keeping a diary spurs Winston toward other rebellious actions (such as adultery) because it is an act of self-expression, and self-expression must be forbidden in societies ruled by governments which rely on total conformity to survive and maintain control of their citizens.