While there are travel insurance plans available for domestic trips, it can be especially important to be covered during international travel because of their expense and remoteness - you may be leaving all your banks, insurance networks, and helpful emergency contacts behind in your country of origin.
Because international trips generally cost much more, it's especially important to have trip cancellation / trip interruption insurance. Whether it's losing your luggage or missing your flight, recouping your losses can become a serious financial hardship without the benefits of international travel insurance.
Medical travel insurance is also important. Many traditional health insurance carriers don't cover medical problems outside of their local networks. If they do cover the medical portion of a trip abroad, they may require you to pre-pay and get reimbursed; and many foreign medical providers won't release you without payment in full - which could run thousands of dollars. One key benefit of international travel insurance is that in foreign countries that lack adequate medical care centers, required medical evacuation costs should be covered. Without coverage, this can run tens of thousands of dollars or more. In comparison to these potential expenses, the benefits of international travel insurance outweigh the cost, making it a wise investment.