The main effect was that the Royal Navy adopted formally a "two-power standard," in which the RN would be required by law to be the equal of the two largest foreign naval powers combined. (In the event, the NDA did not necessarily have this effect in the immediate term.)

The long-term effect was to (to some extent) force the RN's hand in building up its fleet to counter the German naval buildup in the run-up to World War I, though I am of the opinion that the RN would have responded to the German fleet buildup in the same way with or without the NDA of 1889.