REPLICATION SLAVE 常用于建立復制時所需要用到的用戶權限，也就是slave server必須被master server授權具有該權限的用戶，才能通過該用戶復制。
并且"SHOW SLAVE HOSTS"這條命令和REPLICATION SLAVE權限有關，否則執行時會報錯： ERROR 1227 (42000): Access denied; you need (at least one of) the REPLICATION SLAVE privilege(s)forthis operation
The REPLICATION CLIENT privilege enables the use of SHOW MASTER STATUS and SHOW SLAVE STATUS. In MySQL 5.6.6 and later, it also enables the use of the SHOW BINARY LOGS statement.
The REPLICATION SLAVE privilege should be granted to accounts that are used by slave servers to connect to the current server as their master. Without this privilege, the slave cannot request updates that have been made to databases on the master server.
參考文獻：
MySQL 5.6 Reference Manual - 6.2.1 Privileges Provided by MySQL
