There are several cruise-related issues
travel insurance could cover. These include (but are not limited to):
If you have test positive for or are exposed to COVID shortly before your cruise, Disney will (under the current policy) allow you to cancel with a full refund, so you wouldn't need insurance to cover the cruise fare. However, that policy could end at any time.
If you have a different illness, injury, or issue preventing you from cruising (e.g., flight cancellation) you would need to use the insurance to get a full refund of the cruise fare.
If you board the cruise but have an illness or injury during the trip, you could use travel insurance to pay for medical treatment (onboard or in port) and transportation home or to the closest place able to properly care for you. Travel insurance may also compensate you for days missed during the cruise.
If the cruise is late arriving back to port (for example, due to a storm), travel insurance may cover costs involved with changing your flight home and making hotel arrangements if necessary.
If you are purchasing insurance that covers your transportation and hotel on your trip to and from the port, it would be reasonable to check the additional cost of covering the time on the cruise. You can then compare the additional cost to the chances that you would need the insurance and determine whether it's worth it to you.