(http://wiki.dominionstrategy.com/images/6/66/Salt_the_Earth.jpg)(http://wiki.dominionstrategy.com/images/f/fb/Embargo.jpg)
If I can't have it, neither can you!
What do Salt the Earth and Embargo have in common? Each one has a global effect, which in principle affects all players equally. Salt the Earth can make the game shorter for everyone. The Embargo token can curse anyone.
Global effects are everywhere in Dominion. Trashing the last Gladiator lets everyone access Fortune. Emptying a supply pile powers up everyone's Cities, and weakens everyone's Poachers. Trashing an action with Lurker lets anyone gain the action from the trash. Trashing actions gives everyone's necromancers more options.
We can divide these effects into three components: The direct effect (e.g. with Salt the Earth, spend $4 and get 1 VP), the global effect (e.g. a victory card in the supply is trashed), and the timing effect (the global effect begins on your turn, e.g. ending the game by trashing the last province). Usually you get these effects because you want either the direct effect or the timing effect. But when do you specifically want the global effect?
Global effects are most useful when you and your opponent(s) are in asymmetrical positions. For example, if I build towards Provinces, and you build towards Duchy/Duke, I might want to embargo Dukes and you might want to embargo Provinces.
But global effects can have a profound impact on strategy even when nobody uses them. Suppose that a kingdom has two strategies, X and Y. In order to choose between them, we imagine a hypothetical game between a player who chooses X and a player who chooses Y. It could be that Salt the Earth tips the scales towards strategy X, and therefore both players should choose X. And since both players are choosing the same strategy, perhaps neither should buy Salt the Earth.
In games between expert players, global effects tend to appear weak, because experts tend to correctly identify (roughly) the best strategy. This leads to "mirror" games where players have similar strategies. But even when global effects should not be underestimated just because they rarely get used.