The US benefitted from NATO because if the Soviets has overrun Western Europe, there would have been what was essentially a single country (or a country and its satellites which it dominates militarily) stretching from the North Atlantic to the North Pacific, a country that probably would have become wealthier than the US and consequently eventually stronger militarily (if it had the political will to do so, which it probably would have). It was probably worth the expense for the US to have tried hard to prevent the formation of such a wealthier peer, just as it was worth the expense to prevent Germany from uniting most of Europe under its system during WWII (even ignoring the moral reasons for getting involved): being the wealthiest country in the system with the strongest military makes it less likely the country's civilians will get hurt or killed (by e.g. an invasion or a naval blockade).

In other words, there has been a strong streak of national self-interest (correctly calculated IMHO -- at least until the end of the Warsaw Pact and the Soviet Union) in the US's contributions to NATO.