In reply to:
For example: I don't think telling your birthday gives too much information about you, because there are millions of people in the world with the same birthday.
Not true, because a birthday used in conjuction with other information gives quite a lot of info.
I tend to post at various forums and journals, mostly with different usernames. But people who've noticed that some of my interests, etc, sound familar have been able to identify me by my birthday. Maybe sometimes I don't want that to happen.
And some people don't want to give their birthdays, because if people knew how old they were, they might have preconceived notions of them. Some people tend to treat 16 year olds differently from 24 year olds, and differently from 47 year olds, and maybe they want to escape that.
And I used to not want people to know where exactly I lived, either. Country was okay, but I balked at giving the city.
Maybe if I write this now, while drunk, you'll all understand.