Have you ever looked up the word "dado" or "spur gear" in a standard dictionary? You get a circular definition: "A spur gear is a gear with straight teeth." That helps no one.
Unlike flipping through physical indexes, a PDF allows instant keyword search. If you need the term for a specific part of a "carburetor," simply press Ctrl+F , type the word, and jump directly to the diagram. merriam-webster visual dictionary pdf
: It is designed to show the "function, significance, and purpose" of objects through highly detailed, labeled diagrams. Have you ever looked up the word "dado"