The middle example, also produces an accurate circular shape but uses more points than are necessary. In particular the on-curve points, located at the midpoints of the tangents to the curve, add no extra information and might have been omitted. A glyph outline should have points on extrema. Some systems for representing outline fonts require an on-curve point at the point of tangency. It is, however, important to put an on-curve point at an inflection point in a curve.
Inflection points are the point on the curve where the curvature is zero.
An inflection point can be implied if it happens to be at the midpoint of the line connecting the off-curve points.