The body is primarily dependent on oxygen from the lungs and its transport to various organs by the pumping of the heart. Moreover lungs also remove carbon dioxide. If any of these functions fail because of a disease process, external support is required: conventional ventilation for the lungs and vasopressors/inotropes for the heart. Mostly these modalities are helpful, but in some cases high pressure of ventilation and high levels of oxygen delivery to the lungs can cause more harm than good. Likewise, high doses of vasopressors/inotropes can damage the heart and end organs like kidney, peripheries, gut etc.
Putting such patients on ECMO support allows us to turn down the high degree of support thus preventing their side effects while allowing the heart/lungs to rest and heal without hampering the body physiology.