SQL Server 2008

Because the owner is the root of trust. It is the owner that grants, denies and revokes permission on the object
http://www.sqlteam.com/article/understanding-the-difference-between-owners-and-schemas-in-sql-server
http://msdn.microsoft.com/en-us/library/bb669061%28v=vs.110%29.aspx
http://blogs.msdn.com/b/lcris/archive/2007/03/23/basic-sql-server-security-concepts-logins-users-and-principals.aspx logins and users
Logins and Users in SQL Server
http://www.sqlservercentral.com/blogs/brian_kelley/2009/04/21/sql-server-security-basics-logins-vs-users/
https://www.youtube.com/watch?v=c3gPdKaLlkg This is quite good