It defines standalone sections of the webpage, but what’s the main purpose of it? Why is it needed, why say ‘this is a section’?
One of the main reasons is to solve a problem that headings create. Imagine you have a page with lots of headings, and you want a call-to-action at the end that doesn’t relate to the last heading. A section tag around the last heading and the content under it, that doesn’t include the call to action at the end of the page, is a way to say, even though the most recent heading was that, it doesn’t apply to this last line because it’s not in that section where the heading is in. It basically limits the scope of greedy headings.
A good way of separating content more clearly.
Google uses a lot of proximity analysis, to see which words, headings, etc are likely to relate to, and share context with, other bits on a page. Section tags just help it with that job a little.