Rather than cause system crashes, and blue screens of death, they actually fix things that go wrong with the Unix system. For example, they may take the password list and shadow it by coloring it with a black sharpie. They may close off ports that are not in use, so that sailors cannot come and molest the innocent users. They can detect script kiddies and then make them banned from the Internet. Of course, they are hard to get rid of, usually a command like "phoenix command alpha, delete delete delete" used to work, but the Unix Daemons got smarter and changed the phrase.
When overworked, Unix Daemons tend to get very worried and afraid, which can lead to a situation known as 'kernel panic', when all the daemons run screaming away from the operating system, up cables and cords, and can eventually take up residence in the keyboard, sleeping underneath the most unused '~' key. If a unix system has a kernel panic, it is vitally important to pull out all the cables as fast as you can, so the daemons cannot escape.