In addition to the answers that have already been provided, another reason is that if the stamp is has clearly been scribbled on with a biro or marker pen then it is obvious to everybody that it has been used before, thus preventing people from trying to remove stamps marked in this way from their original envelopes so that they can be affixed to another envelope (whether with glue or double-sided tape) and used a second time. Stamps are not supposed to be used more than once, so this process would prevent stamps from being used on two or more occasions, although it does also have the unfortunate effect of making them undesirable to philatelists, as creased, defaced, dirty or torn stamps are considered to be completely worthless.
