The Outer Planes are sealed off from the Inner Planes. The Celestials of the Upper Outer Planes, gave The Seal, to the Keepers in the Inner Planes. While it has power to prevent planar travel from the Outer Planes to the Inner Planes, it has particularly greater power preventing travel from out of the Lower Outer Planes. The Ethereal Planes is the traversing plane between the Inner Planes. You use this to get to the others, sort of like the road to the next plane. the Astral Plane is the traversing plane between the Outer Planes. You use this to get to the others, sort of like the road to the next plane.