AT&T -- Primarily you can use to pay your postpaid wireless bill and In-store available merchandise (excluding gift cards).
Exxon / Mobil -- You can pay for gas or any other purchase in-store (excluding gift cards). You need to have Exxon or Mobil Gas Station to take advantage of this though.
The interesting aspect of Plenti is even if you use the points towards the payment you still earn Plenti points (Although, You earn 1 point/Gallon for Gas which is sort of peanuts, but they have frequent promotions for bonuses).
So, if you do not have any better use (airline partner transfer?); rather than redeeming the Membership rewards points as statement credit it's better to redeem them at 1:1.5 ratio and use it to fill up your gas or pay your postpaid wireless bill with AT&T.