So they can improve the quality of their dessert --- sometimes significantly -- without as much added price as you'd normally think since the chocolate is diluted by so many other ingredients.
To a degree, but the dilution also works against this, depending on how trained your palette is. When baked in a cake I find very little difference between Lindt, Valrhona, some Italian couverture I bought whose name escapes me, and even Cadbury. It's like chefs saying there is no such thing as cooking wine - true mostly if your life's work is focusing on subtle flavour differences.
For ganache and handmake chocolates, however, it makes a lot more sense. I intend to try Callebaut next, $50/kg for Valrhona is breaking me :)