As a person involved in palliative care, I can tell you the simple answer is that palliative care from the right provider for the terminal person can help make the remainder of your days here better than if palliative care were not provided.
A dotted line has recently been drawn separating palliative care from hospice. Technically, a person is not eligible for hospice unless they are expected to pass within 6 months assuming their disease follows the usual path. Many people are on hospice for much longer, years in fact. It depends solely on the individual condition. On the other hand, palliative care per se is simply making things better, irrespective of the terminal diagnosis and time frame involved.
In some cases, people with very serious illnesses that cannot be expected to be cured by organized medicine have trouble getting proper pain control or other types of care that, rather than curing a disease, make life easier. The medical machine is set up to cure and release, not care for the long term. Palliative medicine can fill this void.