Definition of nugatory given by the Oxford Dictionary:
Of no value or importance
Short and simple. So it is basically the same as useless, right? If so, then why is there a need to use it? Why don't we just use useless directly? It seems that the existence of the word nugatory is ... nugatory. In my knowledge, there is no a single word that has an exactly meaning or nuance to another word. There must be a niche market of the word to exist.
So what is the niche market for nugatory?