Adrian spoke about being able to draw vertical/horizontal lines while creating
a geometry. I think the topic goes beyond to the point that, while editing
geometries, it is necessary to combine different techniques, thus the
selection of a feature creation/modification tool should set a "mode", in
order that other context sensitive "modifiers" can be used.
- being editing a line and set an "ortho" mode which only allows horizontal
and vertical lines, or even enabling the drawing of line segments orthogonal
to another one from another feature.
- being editing a line and set the edition mode temporarily parallell to
another, existing one.
- while adding vertices to a line, draw an arc and then continue
adding straight segments.
of course there are much more possible examples. AutoCAD is the most feature
rich app I know on this regard.