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

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.