Final answer:
Illustrations in Palladio's Four Books on Architecture visually explain concepts and designs, making the book more accessible and influential.
Step-by-step explanation:
Illustrations are used in Palladio's Four Books on Architecture to visually explain the concepts and designs presented in the text. They provide visual representations of buildings, plans, and architectural elements, helping readers understand the ideas and techniques discussed in the book. Palladio's use of illustrations made his book influential by making it more accessible and understandable to a wider audience. The visual aids allowed readers to better comprehend the architectural principles and designs, ultimately contributing to the widespread adoption and influence of Palladio's ideas in the field of architecture.