PDF and SVG have special versions of the curveto operator that deduce the control points from less data.

PDF has v and y operators besides the canonical c operator for cubic Béziers: v uses the current point as the first control point; y uses the last point as the second control point.

SVG has t and s operators besides the q (quadratic Béziers) and c (cubic Béziers) operators: both use reflections of existing points as missing control points; they are meant to provide smooth operators.

What is rationale for introducing these special operators? How are they meant to be used?

Are they used in any common format, say font formats?

Does any vector graphics software or library exploit these special operators?