When you open a sketch in 3D, you can start the sketch anywhere. This adds an origin. To use a 2D sketch mode in a 3D sketch, activate a planar face by adding a 3D sketch plane. You can also click
3D Sketch on Plane 
(Sketch toolbar) or to start sketching in a 2D mode. This provides some additional benefits:
-
Inferences. More relations are available, with some restrictions. See Sketch Relations in 3D Sketches.
- Snapping. Only occurs with the geometry in the sketch plane. Exceptions include:
- Snap to a curve that passes through a plane and infer coincidence between the sketched point and the curve.
- Snap to points off the plane when viewing that plane normal to. This ensures that the point is coincident with the projection onto the plane.