No one likes a know it all... especially airline workers!
There you are, Daisy, on line at the airport and your flight gets cancelled because of "technical problems" or "your pilot is drunk." But you don't sweat it because you know your rights, for real this time.
Rule 240 is going to be your secret weapon from here on out because an airline's job is not to turn you away after screwing you, it's their job to keep you from multiple homicide. Should the airline cause the delay (the cause must be the airline's fault, not weather or "act of God." Sorry, Wrathos) you are entitled to some sort of compensation whether it be a transfer to a different city to connect to your ultimate destination or a full refund.
Not all ticket agents are aware of Rule 240 so it's your job to read up, carry a copy of the specific airline's Rule 240, and be polite when bringing the smack down on them.