When set to on this will allow multiple connections from the same hostname. Unix systems quite often have numerous users on the same system. If this option is off only one user from that system will be able to gain access to the server. However, when off it will also limit the number of clones that abusive users can load onto the system. Unlimited (or 0) is the default setting.
/as client WebMaster set multi 20
The reason to keep multi set to 0 (unlimited) is that several people may have a legitimate reason for multiple logins from the same host. The most common being unix systems or cybercafes. But even without this, an extra login is not always a problem. Sometimes someone may want two clients actively chatting. Or they may be running a non-harmful bot. Several people code bots that can run on chat networks and provide amusement. Some typical examples are bar bots that serve virtual food and drink and may say amusing things and trivia bots that can let people play simple games. The people running the bots may then also want to log on as themselves to chat. The benefit to not allowing multiple connections is that you avoid the possibility of clones. You prevent people from wasting connections with unnecessary extra clients. Whether it is worth it will depend on the particular users you tend to have. You can always add a special client class for addresses that tend to need replicas, although this requires more maintenance.