Usage is the study as to why a given stamp was required to be issued, and the ways in which it could be deployed in payment of postal services. Usage is a most logical aspect of stamp collecting, yet historically the least understood by collectors and dealers.
An explanation of the intended purpose for use is provided for each subject featured.
On a macro scale, the topic model can be adopted for virtually any stamp issue, of any country which produces stamps primarily for postal purposes (as distinct from production largely or wholly for collector consumption).
Comments are mine, based upon over 25 years of research in to the concept of usage.
Auction estimate again mine, a guide to market value deduced from observations of public and online auctions, in consultation with specialists, and personal experience gained as a philatelic auctioneer for over 40 years.
This facility is provided to demonstrate that a used stamp barely worth 10c off cover, may be in an entirely different league when still affixed to its original commercial postal article. Observers of U.S. "Prexie" series, specialists in usage of certain Western European countries, Japan, etc, will require no further enlightenment in that regard.

SG 164 – ACSC 178
Stamp issue: 1938 Original perf. ½d Kangaroo
Principal usage: Make-up denomination
Subject item: Solo use Jan 15 1942 at Adelaide, underpays by 1d the intended 1½d Printed matter rate.
Comments: The only solo use of this stamp seen, particularly desirable advertising envelope for exhibitor.
Auction estimate: AU$500