Think about it. If you're using Horn of Plenty to get a Victory card, you're trashing it anyway. The gaining isn't dependent on the trashing, so if you Counterfeit a Horn of Plenty, you get two Victory cards, while only trashing one Horn of Plenty. Add to that the extra card in play from Counterfeit, and the +Buy.
And sure, you could say that "well, I could have just bought another Horn of Plenty rather than a Counterfeit." But think about it some more. One Horn of Plenty with Counterfeit would be the same as two Horns of Plenty, yes. But two Horns of Plenty... now you're getting four victory cards, as opposed to just three. Three - you get six, rather than four... you can see that instead of buying eight Horns of Plenty to snag Provinces, just get four, and one Counterfeit.