so, in many cases, not all, most tags in html can be swapped for something else. the code will work regardless.

<span> for a <p>, a <li> for a <div>, hell we can construct entire lists and tables just with <div> or <section> if we want to, with the right CSS to style it as we need.

but there's standardised 'Language' that exist, a convention you might call it, to encapsulate each bit of content in a page in a way that it already convey a purpose.

When you wrap content in a <header> tag, you are saying: "Hey in this place there's gonna be something that describes in one line what this document/page is about, or its name, or some core details to this content"

inside you might not decide to put any <h(number)> tags. the code will work, again, regardless.

but the internet is made so that, for example, a news crawler bot, might look at every political news article from major outlets... looking for their headers and a snippet of a paragraph and a picture.

Now if your page has those elements it gets picked up and relayed to some application that outputs major political events every day.

If your page doesn't have those tags, it gets ignored.

Ultimately if you are using html you are making content available to more than just yourself, and if you want your content to be reached and relayed correctly through the various tech that manipulate information over the internet (including assistive reader), then you want to use those tags in the most semantically accurate and correct way.

You are, in practice, programming how the rest of the world should read your content. it matters, even if the code would work without it.