Thanks to Janet Klug in Linn's Stamp News for educating me to the difference between a souvenir sheet and a miniature sheet.
U.S. stamp emissions in souvenir sheet form are valid for postage, while in Britain they are not. Those that are are called miniature sheets across the pond.
Among other things, she points out that "...we are naturally drawn to issues that reproduce things such as classic stamps that are beyond our capabilities to own as originals" and that such reprints in souvenir sheet (or miniature sheet) form can be part of a meaningful stamp collection.
Klug's writing under the Refresher Course banner is yet another reason to consider subscribing to Linn's Stamp News, a publication that has been serving stamp collectors for decades.