What is the purpose of putting application to join my club when I can't qualify people requesting to..

In an "open" club, any player can immediately join, without the permission of president or an officer. These clubs are more easy victims of coffee pirates or member stealers. In an "application" club, the president or officer must admit them (unless the player was invited by a president/officer directly) after viewing the player's level and island. Any other screening requirements/communication has be done outside of the game, such as through FB Messenger or Discord.