People write in diaries so that they can keep track of memories. Some write throughout their lives. People sometime have a different diary for when they are pregnant. They want to keep track of what it was like, so the baby will have something to read when he or she gets older. Sometimes you might write in a diary to put together as a book of your life.
I have a diary, and I love it. I've had it since I was five, and it's a gigantic book.