A writer should use synonyms:

Using synonyms for variety is important in writing because it enhances the richness of the text and keeps the reader engaged. When a writer employs a range of vocabulary, it helps to maintain the reader's interest, as repeating the same words can make the writing feel monotonous. Variety in word choice can also help to convey nuanced meanings and emotional tones that might be lost if the same terms are overused. This approach allows for a more dynamic expression of ideas and can better capture the attention and imagination of the audience.

In contrast, while clarity is a fundamental goal of effective writing, using synonyms may sometimes lead to confusion if the words chosen are not precise for the context. Brevity focuses on being concise, which may not always align with the use of synonyms that can lengthen phrases. Finally, confusing the reader would obviously detract from effective communication, making it counterproductive to the primary intent of writing. Therefore, the primary reason for using synonyms lies in creating variety, enhancing the overall reading experience.