Travel insurance typically covers these situations for you and your traveling companions:
Something happens in your life that causes you to cancel the trip (such as illness or injury, job loss, or serious illness in a family member).
Something happens in the world that prevents you from taking the tri[ (such as natural disaster or flight cancellation).
Something happens during the trip that requires significant expenses (such as injury or illness causing medical bills and/or different transportation to get home).
Some travel insurance will also pay your for trip delays, missing or delayed luggage, and being quarantined during a trip.
Insurance policies vary on exactly what situations are covered.
If you have pre-existing medical conditions, you may be covered for those conditions only if insurance is purchased close to the time you book the trip.
You need to determine whether you want to absorb the cost of the trip (and any other non-refundable expenses, such as airfare and hotel stays) if you need to cancel, and whether you can cover the cost of medical treatment and transportation home if necessary. Some insurance is primary and will take care of costs immediately; most is secondary, so you may need to pay out of pocket and then seek reimbursement from the travel insurance company.
You do not need travel insurance, but you may want it. I always buy
trip insurance when travelling from the US to Europe. My first few cruises out of the US I didn't buy insurance, but now that I'm getting older, I insure those trips as well.