Some countries, require that you, as a foreigner, when you travel, hire a health insurance. If not, they will not grant your visa.
Of course, you can travel without health insurance and nothing can happen. But the purpose of paying a health insurance for the time that you are in a foreign country is that it covers the costs of sudden and unexpected illness or injurt.
In the majority of the cases, health insurance covers out-patient care, in patient treatment, emergency dental treatment and repatriation