To understand why descriptive fantasy words are so important, the writer/reader must first understand the role of fantasy. According to the book "How to Write Tales of Horror, Fantasy & Science Fiction," fantasy fiction allows a reader to indulge in a wish being fulfilled or immerse himself into a deep fear. These stories are built upon words -- words that transport the reader through an elf-filled fairy-tale forest or into the dragon's lair in such a believable way that one quickly forgets that these things do not exist outside the pages of a book or on the movie screen.
Words that conjure up strong images even without a story represent some of the primary tools for the fantasy writer. Although many people are naturally gifted with using words, some people must take a few stabs at writing descriptive passages. Having a list of words that immediately evoke a feeling or a strong visual helps. For example, a fantasy writer might use the words bleak, crestfallen or melancholy to convey sadness. These words mean the same thing essentially, but to say that a day was bleak holds much more weight than simply a "sad" day. The aspiring fantasy writer gains easy access to words like these by keeping a thesaurus handy during the rewrite stage of her work.
One of the most famous passages in fantasy literature comes from Franz Kafka's "The Metamorphosis," in which main character Gregor Samsa wakes up to find himself turned into a giant insect or vermin, depending upon the translation. Although the words "insect" and "vermin" conjure up creepiness, it's the use of the words in the context of that famous opening paragraph that make these words more descriptive. Samsa is a man who woke up in his bed after having a night of fitful dreams only to find out he'd turned into a bug, and neither he nor the reader ever find out why he turned into a bug. These elements of normal life -- waking up in bed, the nightmares and Samsa's response to his predicament, including trying to continue on with his workday despite his change -- are what make the words "insect" and "vermin" doubly powerful. The fantasy story that can use powerful words in an everyday context will have the same effect.
Some of the most important descriptive words in fantasy don't convey setting or action, but rather the nature of the characters in the book. The power of a character's name cannot be underestimated. Memorable characters that have appeared in recent novels like the "Harry Potter" series give credence to this; think how reptile-like the name Severus Snape is and how deathly creepy Lord Voldemort has become over the series. A good character name may not immediately draw attention, but the lack of one will. For example, if a person says his name is Count Cuthbert, one might be inclined to believe that the name comes from a humorous fictional horror/ fantasy novel, but would likely be surprised that Count Cuthbert became Count Dracula in later incarnations.