Apart from the ordinary reason of understanding how stamps are used to pay for postal services, their issue by postal administrations is normally by countries, or sovereign states. These express views of those political entities by the design of stamps and postmarks used to cancel them.
Stamps are advertisers for their issuing authorities conveying information about those countries, what they were like in the past. They bear countries’ names (with one exception) and cancelled stamps often reveal where and when they were used.
In Transvaal, numbers, names of places, dates and times used for cancelling stamps are informative. It is a commonplace to say that stamp collecting in one’s youth contributed to knowledge of the world (or world geography) as well as one’s home.