Why Styles Are Important Edit
Using styles in your Writer documents allows for easier revisions, especially for longer documents greater than one page. Direct formatting of headers is not encouraged because it makes it more time consuming if you want to change the look of repeating elements of that document (headers, page numbering, paragraph style).
Using Heading styles rather than direct formatting you can use the Navigator (F5) to select a Section, and then using drag-and-drop, move that subsection to another part of the document. Without styles, one would have to carefully select the text that you want to move, then select Cut from the menu, then move the cursor to the insertion point. Errors could happen and some of the moved text may be lost.
Using Pre-Defined LibreOffice Styles Edit
You should use, whenever possible, the predefined styles included in LibreOffice Writer for your document.
Some styles, such as Heading1, Heading2, will help LibreOffice Writer create a "Tagged PDF" when it comes time to export to a PDF. Writer will build an outline view that is usable in the PDF document, for navigation purposes between Chapters and Sections.