Question:What are the purposes of GRANT and REVOKE commands? 

The GRANT and REVOKE commands are used to manage access privileges. GRANT command needs to assign new privileges to a user or group of users whereas the REVOKE command is responsible for deleting previously granted privileges from user or group of users.

example: 
mysql> GRANT select,insert ON library.book TO jone@localhost IDENTIFIED BY 'open123';
mysql> REVOKE insert ON library.book TO jone@localhost;