From Fodors Travel Intelligence forum (find the full discussion here):
When you book a flight online, at the end, you are usually asked to purchase flight insurance. If you are thinking about purchasing insurance, Is this advisable? Must you purchase travel insurance immediately after you have booked your flight? If you don’t purchase from the site you booked your flight, where do you go???
When purchasing insurance with an airline, coverage will only extend to the time spent flying. Many airlines will only offer vouchers as reimbursement if a flight is delayed or cancelled. If travelers want other benefits such as emergency medical or inclement weather coverage, they should consider purchasing a travel insurance policy.
Generally, there is no need to purchase travel insurance at the time a flight is being booked. Purchasing a policy is only time sensitive when Cancel For Any Reason or pre-existing condition coverage is selected.
The Cancel For Any Reason benefit allows travelers to cancel trips ahead of time without excuse. With Cancel For Any Reason, travelers are reimbursed for a percentage of non-refundable trip costs. This benefit is only available if the first trip deposit is made by a certain date and travelers must cancel no later than two days before their trip departure.
The pre-existing condition benefit provides coverage if travelers need to cancel trips due to medical conditions occurring before departure. A condition is considered pre-existing if it develops or changes within the policy’s lookback period.
Travel insurance policies can be purchased directly through a provider or from a travel insurance comparison site such as squaremouth.com.