Every Draft it drawing has an origin point (X0, Y0) at the bottom left hand corner of the Sheet Border. As the cursor moves over this position the origin Snap is displayed, this means that the point can be selected for the current drawing operation. The most likely use of the drawing origin point is as a reference point for 'Absolute co-ordinate' input. If you want to select a point in a particular command that is a set distance away from the origin make sure that the Snap input mode is current (use to force this). Now the Relative method can be utilised, simply hold the cursor over the origin point briefly and as you move away dynamic X, Y dimensions are displayed (use to toggle between X, Y & Polar). The reference is now relative to the origin so an absolute position can be set.
The Origin point is also the default insertion point of the drawing is inserted as a Symbol into another drawing. If required the Set Insertion Origin can be used to override this, however it is worth noting that it does not alter the actual origin point.