REPLICATION SLAVE 常用于建立复制时所需要用到的用户权限，也就是slave server必须被master server授权具有该权限的用户，才能通过该用户复制。
并且"SHOW SLAVE HOSTS"这条命令和REPLICATION SLAVE权限有关，否则执行时会报错： 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.
参考文献：
MySQL 5.6 Reference Manual - 6.2.1 Privileges Provided by MySQL
