Salvager is a great opening buy, but it tends to stall once you run out of targets. (This is true of most trashing cards other than Steward.) Salvaging Coppers is just not worth the effort; you’re essentially Cutpursing yourself for a +Buy.
This is where Hoard comes in; it supplies your Salvagers with a constant stream of Victory cards. You can grab Estates and Duchies to pick up free Golds, then crunch the Victory cards into cash with the Salvager. And you’ll have so much Treasure in your deck that you can later afford to start trashing your Golds and Hoards for +$6, which is $3-$4 better than playing them (not to mention the +Buy).
Of course, this works just as well with Bishop and Apprentice in place of Salvager, but they don’t give you a +Buy to make use of all those Golds. (This is especially problematic with Apprentice, since you don’t need to draw all your Golds if you’ll just buy a Province anyway.) With Forge, Remodel, and Expand, it’s a little more difficult to make use of the Victory cards, though it could be more viable with +Actions. And Talisman can also substitute for the Hoard, but it’s obviously not nearly as good.
Despite falling behind early on, repeatedly trashing Duchies (and later, Gold) with Bishop lets me catch up in VPs.