Travel insurance does not have to be purchased at the time of booking. The latest opportunity to purchase is the day before departure.
There are advantages to purchasing early. Trip cancellation coverage begins the day after purchase. This means that there will be a longer time that the traveling party is covered if the plan is bought early. The premium cost for most plans does not change based on how far away the departure date is.
Also, there are some coverage benefits that are only available for a short time from the initial deposit date. This can include pre-existing conditions, cancel for any reason, cancel for work reasons or financial default of a travel supplier. By getting a policy early, these coverages are either included or available as a selectable upgrade.
Always refer to the certificate of insurance for details.