Depends. But usually when a person has a low likelihood of recovery and the patient and family would like some help working through the latter stages of the illness both through the physical and emotions . They are very useful !
When you're done... There comes a time in everyone's life when enough is enough. That is when palliative care is appropriate. It can be straight forward such as due to a terminal diagnosis like cancer or end-stage heart/lung disease. Sometimes it is just due to that slow decline that wears down the spirit over time. .