This is because most IRC servers will automatically cut you off if you try text flooding. So CTCP echo flooding spoofs the IRC into falsely cutting someone off by causing the victim's IRC client to automatically keep on responding to a whole bunch of echo requests. Of course your attacker could also get booted off for making all those CTCP echo requests. But a knowledgeable attacker will either be working in league with some friends who will be doing the same thing to you or else be connected with several different nicks to that same IRC server. So by having different versions of him or herself in the form of software bots making those CTCP echo requests, the attacker stays on while the victim gets booted off.
Some bots are hard to tell from real people.
For example, some IRC bots wait for someone to use bad language and respond to these naughty words in annoying ways. The IRC Cops who control hacker wars on these networks love nothing more than killing bots and banning the botrunners that they catch. It's useful to know the response time because sometimes the Internet can be so slow it might take ten seconds or more to send an IRC message to other people on that IRC channel. So if someone seems to be taking a long time to reply to you, it may just be a slow Internet. Again, we are looking at a fairly harmless kind of attack.