The English language is notable for the abundance of synonyms for many words, which enriches prose by offering opportunities for elegant variation, the use of synonyms to avoid repetitive use of one word. Another benefit is that the writer can select a particular synonym to express connotation, the implication of a sense or value for a word.
For example, eager and anxious are often used interchangeably to indicate someone’s anticipation of an impending event. However, eager implies that the person looks forward to the occurrence, while the connotation of anxious is that they dread it. (Unfortunately, this distinction is weakening in modern English usage.)
The careful writer takes note not only of a word’s meaning but also its connotation, because failure to do so can obscure the writer’s intent.
Various synonyms for thin used to describe a person, for example, have a wide variety of connotations. A slender person is one with a pleasing economy of form, and svelte adds a sense of fashionable presentation. A skinny person, meanwhile, is excessively thin, and gaunt emphasizes an unhealthful state. Wiry, meanwhile, connotes a tough, lean build acquired through hard work, while lithe suggests a graceful quality.
By the same token, it’s one thing to say someone is confident, but cocksure is a negative appraisal. A stubborn person, meanwhile, could be described by a proponent as resolute and by a foe as obstinate.
Novice is (or is intended to be) a neutral term, but many synonyms for the word, such as greenhorn, newbie, rookie, and tenderfoot, are uncomplimentary or at least often used to poke fun. (Apprentice, neophyte, recruit, and tyro are gentler terms.) Synonyms can differ in formality, as in the difference between car and automobile, but the difference is often one of value instead (or in addition).
Keep in mind, too, that a single word can have more than one connotation. For example, a person descried as earthy might be simple and practical, or might be unsophisticated or coarse. (Earthy is also a synonym for crude.)
Use of precise terms in fiction and nonfiction alike is encouraged, but be sure the sense you intend is the one conveyed: Consult dictionaries and usage guides, and when you employ a thesaurus or a synonym finder to find a more interesting or more precise alternative to pedestrian prose, make sure you select the appropriate word based on your meaning.