Medical evacuation comes into play when adequate treatment isn't available where you are: serious trauma or illness in a rural area, on an island, or in a country whose facilities can't handle your condition, requiring transport by air ambulance or medically escorted commercial flight to a better-equipped city or back home. It also covers repatriation, meaning bringing you home for continued treatment once stable, and in the worst case, returning remains to your family.
Health authorities push evacuation coverage hard for one blunt reason: it's the single most expensive event in travel medicine. Air ambulance transport across borders can cost more than most people's annual income, it must be arranged and paid for at the moment of crisis, and no public health system or standard hospital bill covers it. Without insurance, families end up fundraising or taking on debt while a patient waits.