6 decades before NATO was formed, Britain was the largest economy and financial empire. The US was up-and-coming but had not yet had its war with Spain to demonstrate their arrival. Nato was formed "To keep the US in, the Germans down and the Russians out" (per Ismay). The first clause is now inoperative. The second clause is being reversed by the various Euro states and the third, unfortunately, remains.

There was also a benefit to the US maintaining NATO - it could nudge/encourage/guide other countries into doing things it wanted done (such as Afganistan). This soft power is being discarded with NATO.