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 be connected in several different ways 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. But just about every Unix IRC program has at least some CTCP flood protection in it. Then there is the old standby, ping flooding. There are several programs that you can run with your Unix IRC program. Examples are as LiCe and Phoenix.