We must bear in mind that sectioning elements are essentially DIVs in sheep’s clothing. Some have default styles like lists and blockquotes, definition lists and paragraphs, etc. SECTION and ARTICLE are nothing more that DIV with a different name. Toss in ASIDE, MAIN, HEADER, FOOTER, etc. and we conclude the names mean something to both user agent and document readers (both human and assistive technology).
The purpose of markup is to document the document. In the past we would use generic containers (eg. DIV) to house the document content and give them IDs or CLASSes to distinguish their role.
What’s more, in the past it was fairly assumed that more than one H1 in a page was faux pas. HTML5 opened the flood gates on how many topics could be covered in a single document bringing it more in line with multimedia (as in, magazines) in presentation.
The new paradigm permits multiple unique topics in one page. This means one H1 for each topic. Since it never makes sense to have more than one H1 in a topic, we have sectioning elements to contain them.
We are taught in grade school the importance of an outline when writing an essay. A web document is no less dependent upon the same concept when structuring one. Sectioning elements are the markup we employee to build the document within the given outline.
SEs understand document outlines, and give importance to a page if said outline is discernible.
Bottom line, if one has never written essays, then writing a web document is unbridled. HTML being so forgiving, who knows what kind of GIGO will get past it? A well formed document passes the scrutiny of validation and accessibility grading and (one would hope) can be trusted to render in all devices.
There is no set pattern to follow that will optimize a page. It’s our document, and we have an idea of how we want it to present. Now it’s up to us to create that document outline, and then fill in the pieces. Once the markup is written and validated, then we can move on to presentation. It should be a breeze if we have a well formed document.
Finally, a SECTION can have multiple ARTICLEs, and vice-versa. We are the architects, the publishers. We define the roles.