The main purpose of the voucher is to save a little money (probably a few dollars per person). For some, they like knowing they'll be saving a few dollars and having some of their vacation costs paid upfront. The voucher usually covers the price of the meal, tax, and gratuity. The amount you save depends on which voucher you get, where you go, and for which meal you're going. In general, you're best bet to save some money is to use the voucher at dinner.
As the PP said, you still need to make a reservation regardless of payment method. Reservations open 60-days in advance online and over the phone. While reservations are pretty easy to come by, I would still recommend making one if you know you want to eat at a specific place and time. I've been there when the CMs were turning away walk-ups or telling them it was a long wait (>1 hour).