Back Stamps / Advertising On Stamps

I found a thread mentioning this topic from 15 years ago. I did not want to reply to such an old thread. Seeing that dealer's stamp in a recent thread made me become more curious about these ones from my collection. These advertising / back stamps are for Errington & Martin, who were a stamp album and stamp dealer of the late 19th century until the early 20th century. I have some questions.

First, is such a thing in any way interesting, cool, or does it add anything to the stamps collectability?

Second, are they very common? These are the only ones I have found.

Third, do they add to or detract from the stamp?

(I cannot add images of my stamps at the moment as a result of the failings of the image optimiser)

The Errington and Martin back-stamps turn up quite often - no effect on value, and they're usually on common stamps.