Sometimes proofs are printed in different colours to assess the effects. Since 1970 proofs are no longer offered to collectors in the Netherlands.
Seen in a topical way proofs are the philatelic items. It does not matter if these proofs have lead to a definitive issue.
Proof of a machine franking stamp
A proof which pictures topically the same as the corresponding stamp will be in many cases be of more importance (postal processing, rarity) than the stamp (printed in a millions edition).
It is not meant to fill
whole pages with easy available proofs, because that will break,
inconveniently, the topical development.
by Jan de Crom
taken from "The Postal Bee" 2/2008
Contact Group Bee Philately