Why Service accounts in Linux and Unix Systems?

I have a question regarding Linux and Unix non-user service accounts. We know there are user accounts in Linux , what are (non-user) service accounts such as daemon,news, bin, etc used for? $cat /etc/passwd | more reveals all the user and non-user accounts. Why has bin has been assigned service account? Is this by design, or it serves a special purpose? **Last question** Is this true in Microsoft operating system environment( Server 2003, or 2008).?