Semantics & Accessability
You should give the right meaning (semantics) to every small or large piece of content on your website. This is important for assistive technologies like automated translation, spell-checking or a screen reader for blind people. For example, smart speakers and screen reader use text-to-speech to read the text and other visual elements of website aloud to the user. A smart speaker needs to know whether a piece of text is a heading, a sub heading or a paragraph so that visually impaired users can navigate through the website without seeing it.
Screen readers announce the heading level (h1-h6), so users can decide to skip through sub-headings and let the tool read aloud starting at a particular heading. While reading, the tool might make a short break between paragraphs. Buttons and links to other pages are announced differently to indicate the functionality or where the link leads to. So, structuring your web pages with proper headings is important.
So, all these different content types should be described by an appropriate tag. It's your goal as a programmer to make a website as accessible as possible and provide a great user experience, no matter what device or impairments the user has. So, semantics (the meaning of words) and accessibility (A11y) are important. Even able-bodied people can use smart speakers like (Amazon Alexa, Google Assistant, etc.) and rely on spell-checking when writing a comment. Therefore accessibility matters for everyone.
Further Reading
General information
Tools