meaning - When to use "nugatory"?

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?

Answer

Nugatory is a quite formal word used mainly in Law meaning 'not valid' or formal speech.

Example:

• 
  

  The clause excluding negroes and Chinese from the suffrage has never been repealed, although it has been rendered nugatory by the Fifteenth Amendment to the Federal Constitution.

  
  


• 
  

  We may be forced to conclude that the interest of the whole affair, so far as authentic history is concerned, is really nugatory and that the romantic imagination has created a mystery in a fact of no importance.

  
  

(sentence.yourdictionary.com)

Nugatory , Vocabulary.com also suggests a jocular use of the term in current usage:

• An adjective meaning “trifling, of no value,” nugatory comes from the Latin nugatorius “worthless, futile,” which in turn came from the also Latin nugatory — “jester, trifle.” It’s a word you probably don’t hear too often, but it’s a fun and descriptive one to use. Describe something with no force or importance as nugatory. "Whether this rug is red or green is nugatory to someone who is colorblind."

Nugatory(adj.) (Etymonline)

• "trifling, of no value," c. 1600, from Latin nugatorius "worthless, trifling, futile," from nugator "jester, trifler, braggart," from nugatus, past participle of nugari "to trifle, jest, play the fool," from nugæ "jokes, jests, trifles," of unknown origin.

• Its usage has been decreasing consistently in recent decades as shown in Ngram.