The CTCP command performs certain client specific functions on the IRC network. CTCP commands can also be disabled by the user. So you could get no information back from users when you initiate certain CTCP requests to them. It measures the amount of lag time for the message.
Excessive lag can make communication difficult.
I guess it might be useful if you wanted to know what time it was in Australia, if your were talking to an Aussie. Can be useful if you want to know if things like DCC, ping, or finger are active on their system. The DCC channel is not subject to lag and is secure because it does not use the chat channels to transmit information. Some files could contain a virus. For security reasons, you should always make a virus scan on a file before you execute it.