Reading the blank first gives the reader a preview of the most important concepts

One answer could be subheadings. In many textbooks, concepts are organized into units, chapters, lessons, and paragraphs. A textbook may have only a handful of units divided into about 10 chapters. Each chapter may have several lessons. Each lesson will have a title. Paragraphs within that lesson are grouped according to concept. Each new concept is usually introduced with a new subheading. Reading the subheadings can give the reader a quick summary of the concepts that will be discussed in the lesson.