If you are covered by international health insurance or a local medical plan, your policy may have some limitations, like an area of coverage restriction (plan covering only your country of residence, Asia or worldwide excluding USA) or existing coverage may not be enough to cover medical fees in the country you will visit, so buying a single travel insurance for the coming trip will allow you to cover the gaps in your current policy. Also, you may not have included the evacuation part in your international health insurance or local plan so travel insurance will be needed when travelling abroad and cover the costs of medical evacuation to the nearest hospital or back to your country of residence.
Even if you have back home a comprehensive coverage and all your medical expenses overseas are covered, you may consider travel insurance for the accident insurance part, or coverage for lost luggage and travel delays which won’t be covered in your existing health insurance plan.