Visual Hierarchy
The easiest way to give credence to the words you write is to make them appear in a visually descending format. Information appearing in larger type is more likely to be perceived as important. It provides structure and order to the information presented, giving it a sense of planned and intentional design.
Additionally, people are drawn to the information they deem most important. This is a fundamental principle of human psychology. If something appears visually larger, it is worth more weight to the reader. This is why we tend to read from the largest font size to the smallest font size.
This is also a useful tool for accessibility. Screen readers will always read from the top-level heading down to the smallest subheading. If you want to ensure that your content is available for users with visual impairments, it is critically important to pay attention to where your headers like to sit.
When unstyled, HTML gives you this hierarchy straight out of the box. You can use CSS to override this default, but it is critically important to understand why it is useful. You may not need all 6 heading levels, but having a clear visual delineation between sections by physical size is useful for users scanning through your page.