Cancelled/delayed flights can mess with visa/immigration, mess with events (imagine being the speaker to a conference/doing a tour and then not being able to show up because your flight was delayed or cancelled), and even hotel rooms (some places will void your reservation if you don't show up). Also if you get stuck in an area for an extra day it's effectively the same as having gotten the date wrong on a hotel. I've gotten screwed due to a soccer playoff in the EU once happening the same weekend I was supposed to crash at a friend's place
IDK, they both suck.