Empties are just that, empty objects, inasmuch that they have no ObData datablock. Hence, they neither have any Edit mode nor any shape.
But they behave exactly as other objects, you can translate, scale, rotate, …, them. Of course, they are invisible at render time.
So why use objects that have no shape and are not rendered at all? Well, precisely for these reasons! As objects, they can be parents of other, “real”, objects. They can also serve as hooks for mesh vertices or curve/surface control points, and so on. In a word, they can be invisible “steps” in objects’ relations.
They also can serve as placeholders for some things, like force fields.
By default, an empty is materialized by its three local axis. But you have different choices, regrouped in the Empty Drawtype drop-down list of the Link and Material panel:
- Arrows – The default, three arrows along the positive directions of the local axis.
- Single Arrow – Only one arrow, along the local Z axis.
- Plain Axes – Just the three local axis, forming a sort of 3D cross-hair.
- Circle – A circle, on the X-Z local plane.
- Cube – A cube…
- Sphere – An (UV) sphere…
- Cone – A cone!
The Size setting just bellow controls the size of the visualization, not the scale of the object!