Document structure is significantly important to those with certain types disabilities. The fact is that the headings you created inside of a Microsoft word document generate a table of contents for those who are blind or have a learning disability that effects reading. It is a good practice to use the styles, as the markup creates tags that generate this table of contents and it only takes a couple seconds to do.
Home ribbon> Styles
The red and blue rectangles show the first two level of headings. The title of the document, should be heading 1 and the subheadings should be logically added. Most find three of four headings levels are adequate. The headings break a Word document into segments that are easily and quickly navigated. The structure may seem invisible to you, but the assistive technologies used to read the file find each of these headings and sound them out to those with print disabilities. The circle shows how to arrive at more style listing including level 3 and 4.
Rationale: Screen reader users navigate through a document using heading styles. Styles also convey emphasis and semantic meaning, where formatting (i.e. bold, or size) does not.