[MajidCS]

Because not all people in Twitter you know, and you can not get their mail.

When there is a famous person, and you want to send him a private message, what do you do?

Are you should follow everyone they follow you until they send you or you send him a private message?

You can write a long letter in Twitter.

You can not attach a file in Twitter

If the message does not want to be seen by anyone and you can not put it in public.

Private tweets will help you to do that.