This is useful to simplify precision modeling, David.  If you aren't familiar with how something like this could assist your workflow, then it' probably best not to worry about its application at this point in your learning process.

Every face on every object is aligned to a vector direction; otherwise known or referred to as a face or vertex normal.  This "normal" direction dictates how the transformation manipulators are positioned when you select an object's element (such as a face).  A face on a cube, for example, will point in a different vector direction than the face of a tetrahedron (as shown in Cybermonk's screencap).  This trick allows you to copy the vector direction from the transform manipulator of one object's element selection, and use it to move, rotate, or scale the elements on another object.  This is useful for certain modeling tasks, because it allows you to lock the movement of a selected element to some other object's vector normals direction, and move it along those axes.

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