Most philatelists, and especially most stamp dealers, see a dichotomy between traditional stamp collecting and the collecting of stamps on covers, which has come to be called postal history. Years ago, the best dealers and collectors of covers and postal history came up from the ranks of traditional philately. It was only after someone knew the stamps of a particular area and had collected and studied them for years that they would begin to study the use of those stamps and add covers to their collections. This fundamental way that most collectors began their hobby began to change around 1970 when collectors and dealers started to specialize in covers without any real knowledge of the stamps that franked their covers. This change occurred because increasing philatelic popularity had made prices so high that creating traditional stamp collections was too daunting for many collectors, and they moved into the hobby as cover collectors before having any real grounding in stamps.
The problem with becoming a postal history collector or dealer without having a good knowledge of the stamps that you are collecting on cover is obvious. Cover collectors are much more accepting of mediocre quality than are stamp collectors, partly because it is very difficult to get all of the pieces lined up in perfect condition- cancel, stamp quality, centering, and the cover itself without faults- but also partly because cover collectors don't know the current collector quality standards for stamps because many of them have never even been traditional collectors. Further, knowledge of stamps is a precondition for knowledge of covers, postal history, and how stamps were used. To study only postal history without reference to stamps would be like studying medicine without reference to the body. Lastly, the migration directly to postal history without a sound basis in stamp collecting has meant a separation in language and interest between the two broad groups of philatelists. And our hobby is not large enough for two groups.