Welcome to the community!
Congratulations on getting a new phone! You do not necessarily need to use the new SIM card. It is provided in case your new phone requires a different sized SIM from your current phone. If both of your phones use the same sized SIM, you can continue to use your old SIM. If your new phone requires a different sized SIM, you can update your SIM via My Account. You should note that only one SIM can be active on your account at a time. If you update your SIM, your old SIM will no longer be active.
Hope this helps 😀