Credit and goodybags are 2 different things. You can choose to simply have credit on your account without a goodybag, and use the credit to pay for occasional messages, phone calls and data use. (You will also need credit if you're travelling where a goodybag does not work, e.g. USA)
I would suggest you weigh up how much you use your phone, as if you are planning to browse Facebook outside of the house you will probably be best to consider a goodybag, rather than simply having credit on your account.
There are 2 ways to top up to use service on giffgaff, either goodybag (free call, text and data allowance valid for 1 calendar month) and other is airtime credit (can use any service and can last until used up).