There is a lot of information contained on a cover, information lost when the stamps are removed (torn off, soaked, clipped). The lost information results in the inability to answer basic question about the use of interesting items such as your war tax label's date, place and nature of use.
One should give consideration to such circumstances before destroying a postal history cover to collect just the stamps. Many times it does not matter, other times it can; one needs to choose wisely. As one's knowledge in philately expands the choices often become easier to make.
Of course, back in the early days of collecting soaking is what was done.