Airlines are supposed to pay compensation, re-book flights and provide refunds if flights are delayed for more than three hours, with the amount of compensation rising as the delays pile up.

Source: https://nationalpost.com/news/politics/canadian-transportation-agency-budget-cuts

Suddenly you have to re-book your flight, rearrange any hotel accommodations you may have had set up, car reservations, pickups from family members, and hopefully not lose any vacation time that you were supposed to be spending on a beach.

Source: https://wnbf.com/ixp/10/p/buffalo-airport-cancelations/