You need one if you do not have an active sim at the moment. Here are what the goodybags give you https://giffgaff.com/buy/sim-only-plans
You can cancel them at any time after purchasing the phone.
Sorry i still dont understand ,I currently have an 02 contract which i pay a monthly fee for .I dont have to pay any extras .If i get a contract phone from giff gaff for £17.13 do i also have to pay a monthly amount for a goody bag ?
Before you procede any further, take a look here Credit & goodybag confusion to see how airtime credit and goodybags work.
If you don't have an activated giffgaff SIM at the moment then check here http://community.giffgaff.com/t5/Getting-started/Becoming-a-member/ta-p/10441077 for more information on becoming a member to enable your phone purchase.
If you're still not sure of any then just ask again.