The component may throw an exception while executing and the site-C-ing will gladly pass it to the handler component which will give the user appropriate output (unless it throws an exception itself, of course). Here you will see an example output provided by the handler bundled with the site-C-ing.
Suppose you have a component, which at some point throws an exception. Of course, site-C-ing will catch the exception and pass it to the handler, specified in the configuration file, which will produce some nice, human-readable output. Well, you may not wish to give out all this information in the production environment, so you just put in your configuration file some different handler, which just gives user a friendly yet lame excuse.