Some people write in diary's because one can write down everything that's going on in their life. Not everybody likes to talk about how they're feeling or what they're going through to family, friends, or anybody else. So, they write it down in a diary. It's kind of like talking to somebody, which is why people might do it. In conclusion, you can write everything out in a diary, which can give some kind of therapy to those who are going through a lot.
Oh yeah, when I was younger I used to have a journal and would write about stuff so when I got older I could go back and read the things I wrote. It can also help you remember all your memories. It's a good idea. I need to start doing that again.