It seems that the NW Visa card wants to stay in touch with us, so they are offering 500 miles to register your email address with them.
It makes sense to me as they are getting pushed out by the Delta Amex cards in the merger.
It looks like you must be an existing NW Visa card holder to qualify.
It took two minutes. It may seem silly, but all the offers around these days add up to free tickets. Take the time. It will be worth it.