You may purchase the TiW card at guest services as long as you have an active AP. It is good at virtualy all Disney operated table service restaurants, lounges, and counter service in AK and value resorts.
An 18% gratuity is auto added at TS and lounges, not the few counter service venues that accept the card.
Basically, you are saving the amount of the tip and some tax, assuming you'd normally tip in the 15% to 20% range. But that can add-up quickly.