Final answer:
Johnson's dictionary is notable for its extensive literary research, reflected in quotations that illuminate word meanings while providing insights into various fields of study.
Step-by-step explanation:
The excerpt from Samuel Johnson's preface to his dictionary illustrates the significance of his work primarily because it reflects considerable literary research. Johnson indicates that he chose quotations to evidence the meanings of words, ensuring they also provided additional value such as philosophical principles, historical facts, and poetic descriptions. His aim was to make each quotation serve a dual purpose: expanding upon the understanding of a word while offering insight into its use across various academic and creative fields, including divines' exhortations. This method of dictionary compilation signifies a comprehensive approach that would not only elucidate the meanings of words but also enrich the reader's knowledge across multiple disciplines.