What is the use of FLUSH PRIVILEGES statement in MySQL?

Actually, we need to perform flush-privileges operation to tell the server to reload the grant tables. This can be done by issuing FLUSH PRIVILEGES statement or by executing a mysqladmin flush-privileges or mysqladmin reload command. FLUSH PRIVILEGES is really needed if we modify the grant tables directly using such as INSERT, UPDATE or DELETE, the changes have no effect on privileges checking until we either restart the server or tell it to reload the tables. But, Privileges assigned through GRANT choice don't want FLUSH PRIVILEGES to take effect - MySQL server cannotice these changes and reload the grant tables instantly.