Why do customers ask not to post your work in the portfolio?

Question

Portfolio

For what reason do clients not want to see the completed work in the portfolio, because this is additional advertising for them

Answer 1

In practice, because then they will lie to their customers that they did it themselves, or that it cost much more directly.
In theory, in the context of the mythical NDA (as in the Middle Ages - the eyes of the masters were gouged out and executed so that they could not repeat their masterpieces), but in fact, damn it, I subscribe, I saw hundreds of cases from the "practice" described above, and none one of this "theory".
I believe that cases of "theory" do not really exist.