As the PP said, historically you've been able to use your snack credits to purchase items at the F&W booths. Disney usually does not announce this in advance, so we have to wait and see when the first signage goes up on the booths to see if the little DDP logo is on it!
However, you cannot use the credits to purchase anything with alcohol in it (that includes any food item with straight alcohol in it, like the ice cream with grand mariner last year ).
If it's still the case and you can use your snacks credits during the Festival, do so! It's totally worth it! For sure, the best snacks I had on the Dining Plan!