Styles define the structure in a document and make sure that a PDF contains the correct tags. To read the text in a document and present it in a way that makes sense to a user, a screen reader requires that the document is structured. Such tags in a PDF define the reading order and identify headings, paragraphs, sections, tables and other page elements.
Therefore, it is important to use styles in a document to determine a structure that ensures a proper reading order.
Branded styles in Word 2016 ribbon.