Using the number on the back of the band, you can link it to your MDX profile yourself. Or they can do it at the resort when you receive it. Any band you have linked and assigned to you will work on your account. Nothing is actually stored on the band. All it needs to do is know which account to read from.
You may want to remember that once a band is linked to a MDX account, it can be deactivated by the account holder, but it cannot be removed. So you can't link that F&W band to your account and then decide you will give it to your friend who will be coming to F&W later.
As noted there are several uses for magic bands:
Park entry (if you have tickets linked to your MDX account)
Redeeming Fastpass Plus reservations
Access to your resort room (if you have a Disney resort reservation linked to MDX)
Charging back to your resort room account (if you have a Disney resort reservation linked to MDX with charging enabled)
Paying for food or beverage items using dining plan credits (if you have a Disney resort reservation with a dining plan added)
You don't need them for anything else. You don't need it for restaurant reservations - you need it to present payment at the restaurant if you are paying via room charge or dining credits, but you will not need it to confirm that you have a restaurant reservation.