Tolkien's legendarium is a work of fantasy, and one of the main aspects of fantasy is the idea of enchantment: transporting readers into a different realm through language and storytelling. It makes sense, then, that Tolkien incorporates monsters into his work. They are necessary for the weaving of the spell over the readers, while also providing characteristically monstrous levels of menace. Tolkien, being a lover of language, is able to modulate this menace and each monster's apparent evil by changing how - or if - they speak. In fact, monsters' purposes can be clarified by examining their speech patterns, solidifying their place in the narrative and Tolkien's legendarium as a whole.
Shelob is a rare case of monster, as she does not speak. However, this adds to her character. Evil is often associated with the unknown, and the fact that Shelob does not speak adds a component of the unknown to her existence, therefore indirectly associating her with evil. This is compounded by Tolkien's initial descriptions of her:
She had "two great clusters of many windowed eyes... Monstrous and abominable eyes they were, bestial and yet filled with purpose and with hideous delight" (The Lord of the Rings, 720)
Much of Shelob is left unseen until the hobbits leave her cave: until then, she is just a silent collection of eyes. The silence, as well as the focus on the eyes, links her to Sauron, who is represented by an eye and who does not speak directly to anyone throughout The Lord of the Rings. Sauron and Shelob do not collaborate, yet they are connected in that they are both linked to older sources of evil, such as Morgoth and Ungoliant. Therefore Tolkien manages to create a truly terrifying spider by purposely voiding her of her physicality and her speech, connecting her to the major villains of the Middle Earth story and presenting her as a larger force of evil.
Unlike Shelob, who embodies evil and darkness, the Mirkwood spiders serve more as plot devices than as a reminder of the cosmic struggle between good and evil. This has much to do with the style of The Hobbit, which reads as a more traditional fairy story than The Lord of the Rings. This also has much to do with the fact that the Mirkwood spiders can speak. Tolkien describes their voices as "a sort of thin creaking and hissing" (The Hobbit, 168), with the words creaking and hissing both creating sensory enchantment for the reader. Their speech patterns are slightly colloquial, demonstrated by words such as "a-struggling", "bee-autiful", and the shortening of "them" to "'em". However, Tolkien does little else to distinguish their speech patterns, and as such they are less memorable. The reader knows what they look like, how they sound, and what their motivation is. Therefore, there is little left unknown, and, as is the case with Shelob, the unknown is what truly makes a monster.
The trolls in The Hobbit are similar to the Mirkwood spiders in that they are simple obstacles to the company; they are not truly evil. Like the Mirkwood spiders, their language is colloquial: Tolkien describes it as "not drawing-room fashion at all, at all" (The Hobbit, 44). They turn the words "you" and "your" into "yer", and they shorten "them" to "'em", but that is the extent to which Tolkien distinguishes their speech. Its simplicity demonstrates the trolls' stupidity, and, since they appear early in the narrative, this allows the reader to adjust to the notion that there are trolls and other monsters in this world. Therefore, the main purpose of the trolls as demonstrated by their unremarkable speech may be to prepare the reader for the more compelling monsters later on in the book, such as Gollum and Smaug.
Gollum has one of the most fascinating speech patterns, if not the most iconic, in Tolkien's legendarium. The sibilance of "preciousssss" and the occasional "ssss" add to the enchantment of the story and introduce Gollum as something truly different and unknown - one of the hallmarks of being a monster. Similarly, the inclusion of "my precious" consistently adds to Gollum's character, as it is a verbal representation of his enslavement to the One Ring. Gollum's trademark sound - gollum - is also unique, as Tolkien chooses to include it in the actual dialogue instead of simply saying "and then Gollum made a horrible swallowing noise in his throat". In doing this, he has created a form of dialogue that is integral to Gollum's character, just as Gollum is integral to the overall story of the Ring. He has also crafted a speech pattern that reveals much about Gollum's nature and character: just as Gollum's speech is a combination of the evident and the unknown, so he is a combination of monster and hobbit.
Similarly to Gollum, Smaug's speech patterns reveal much about his character while still preserving his menacing nature. Out of all the monsters, Smaug has the most elevated form of speaking, demonstrating his intelligence. Unlike the Mirkwood spiders or Gollum, there is no indication of how he sounds when he speaks, so the true fear comes from his words. This is a clever move on Tolkien's part: he is forcing the reader to imagine how Smaug sounds, which is in itself an aspect of the unknown. Smaug doesn't have any defining verbal tics like Gollum, but his words are extremely calculated and serve to discover who Bilbo is and how he is linked to the dwarves. For example, he asks Bilbo:
"Has [it] occurred to you that, even if you could steal the gold bit by bit ... you could not get it very far? Not much use on the mountain side? Not much use in the forest? Bless me! Had you never thought of the catch?" (The Hobbit, 237)
Smaug's dialogue is written in an almost linear progression, making him appear as if he is deducing Bilbo's purpose. Similarly, the interjection of "Bless me!" adds to the image of him being crafty and manipulative, as he is trying to coerce Bilbo into giving up what he considers to be a useless quest. As Smaug is the last monster Bilbo must face, he is also the wiliest, and his cunning language and linear deduction reflect this level of sophistication.
Tolkien's ability to distinguish characters through their speech - or lack thereof - demonstrates the versatility of his monsters, and also justifies their existence. Just as each monster has its own specific dialect, they have their own specific purpose, be it as obstacles for the heroes or as the main purpose of the quest. While some critics may have problems with the monsters' presence, they cannot deny that Tolkien's stylistic approach to them is impressive, and that it fits each monster's unique personality and form. Would Smaug's words sound as menacing or as calculating if they were coming from a man? Would Gollum's speech be as frightening or as intriguing if he were a human? The answer is no: monsters and their speech are essential to Tolkien's stories, as they enchant and build the world of Middle Earth, as well as move the story along.