Generally it was for when the game was less advanced, and was much easier to exploit. This meant a lot of tactics (Mr Houghs etc, or even back a long time ago, Diablo) relied not on any sound footballing or tactical basis, but simply by "breaking" certain parts of the match engine. This meant that the tactic would pretty much work straight away, but also that you didn't really need to shape your players around a tactic, because pretty much anybody would work in it. Hence the term "plug and play".