Before the late 1970s (and the advent of Federal Express and overnight delivery), the only way to move documents from one place to another was by post. Throughout the late nineteenth and twentieth century, the post was not only the fastest way to move documents; it was the only way. Ordinary postal delivery was fast. By 1900, mail from Toronto to Ottawa or Montreal was usually accomplished overnight. But businesses required even faster communications, and the Canadian Post Office (and other postal agencies worldwide) usually provided it.
Special Delivery is a service where mail is expedited. In Canada, delivery from east to west coast was only two or three days. In the period between establishment of transcontinental railroad (1885) and transcontinental airmail (begun about 1920 and pervasive by 1930), special delivery service was actively used in Canada, even though the fee for the service was five times the traditional postage.
Special delivery stamps are interesting for another reason—they are testaments to the geography of the countries that issued them and the technology of the time. Countries like Great Britain and France which are densely populated and had early rail systems throughout the country never issued Special Delivery stamps as mail delivery was always a one or two day affair. Countries like the United States and Canada which had a few dense population centers separated by thousands of miles of comparative wilderness, issued more than 25 such stamps between them. Commerce and political considerations required that these two large countries connect their distant regions with rapid communications, and because of this the US and Canada issued the most Special Delivery stamps of any country in the world.