But I thought you said…

Yes, if couverture is for coating things, then why does the Sacher Torte recipe call for it in the sponge? The only reason I can think is because it contains quite a lot of fat, and fat creates the sensation of moisture — necessary in a cake that’s known for being a bit on the dry side. You can substitute another (cheaper) type of chocoalte in the recipe if you plan on using cake syrup.