Why am i better off with this mortgage offer?

I have 8 years left on a 15 year mortgage 5.12 percentmortage,and was offered a new 15 year at the amount i owe at 4.50 percent why would i choose the new offer.To me there is no benefit except the intrest rate but i would be paying more intrest and extend my mortgage 7 years to 15 years if i accepted the offer