Newtons explanation of gravity as action-at-distance is impossible without the presence of some intermediary medium between the bodies acted upon. But experiments show that the postulated ether does not exist between the planets and the sun.
The concept of field is necessary. The justification for this concept is somewhat arbitrary but it is useful as an aid to understanding how electromagnetic waves [light, radio, micro, etc.] are transmitted through empty space. A gravitational field can explain how astronomical bodies are able effect each other instantaneously as well as how the earth acts on a dropped stone. The earth produces in its surroundings a gravitational field. It is the field that acts on the stone and produces its motion of fall.
A gravitational field, unlike electric and magnetic fields, produces an acceleration which does not in the least depend either on the material or on the physical state of the body. For instance, a piece of lead and a piece of wood fall in exactly the same manner in a gravitational field (in vacuo), when they start off from rest or with the same initial velocity.
[Through a simple exchange of terms in Newtons
formula, F = ma. Einstein shows how the idea of a gravitational
field explains why the inertia of a body is equivalent
to its weight.]
The same quality of a body manifests itself according to circumstances as inertia or as weight (lit. heaviness).