Guess pretty good example is Donner's mid flag. With a good smoke you blind enemy snipers and usually their rushing in general, with a horrible smoke(i'm good with these), you blind all your teammates, cause rage and be executed for treason.
I think mostly a good smoke is far closer to the enemy than it is of you, then it covers more of the enemy's area of sight, and usually a good practice is to nade the friendly smoke with HE-ones too, to cause casualties and suppressive effect to the enemy.