Let me give you some reasons (just to make things clear, I am neither a
travel agent nor an insurance broker, just someone who's traveled A LOT over the years).
1) 3 days before the trip, one of your traveling party suddenly needs an appendectomy. Disney will, at their discretion, sometimes let you shift when your trip was to be, but not 100% of the time, & the airlines? Good luck!
2) While on your trip, someone gets very sick & needs to get brought back home RIGHT NOW to go to the hospital.
3) On your way down, you go to Orlando, your luggage goes to Ottawa.
4) On your way back, your connecting flight in Atlanta doesn't manage to connect and you're stuck in the airport for a day (or two) & the airline, saying it was weather related, doesn't do anything to help either get you home sooner or pay for you to stay somewhere.
5) On your way to the airport, you get stuck because of a traffic accident that you're not in but you miss your flight, and your flight is trying to connect you to your sea portion of your land & sea vacation with Disney.
Depending on the insurance that you get (some cover preexisting conditions, some don't), it may cover many or all of these possibilities. It will pay for your cancelled trip, your missed connection hotel stay, etc, etc, etc.
Will you get the value out of the insurance every time? God willing, NO! Will it be worth it the one time that any/all of that DOES happen? Depends; If the insurance costs $100, and your trip costs $3000, it's probably worth it.
Any questions, ask away!