What are the practical benefits of having just one <h1> per page

Question

It is a recommendation to have just one per page, even validators print out a warming if there are more, but other recommendations contradict it like:

1. Any rank can be used as the heading of an explicitly-defined section, although this practice is not recommended.
2. Never skip heading order

and there is also the sectioning elements like aside which's outline shouldn't affect the page's outline.

It is pretty confusing, so at least I want to know how does it make better to have just one per page.

Answer 1

You didn't state who makes the recommendation but the specification itself says no such thing. A heading begins sectioning content and there may be more than one sectioning content on a page. However, what you are asking about, is the "document outline" and it does not exist in browsers today. In particular note this reference from one of the HTML authors and ARIA:

The HTML5 Document Outline is a dangerous fiction

It is dangerous because it can lead unsuspecting developers to think that using the nesting of heading elements in sectioning elements actually has some effect for users who consume heading semantics. Overwhelmingly the opposite is true. For example If you code a heading as a h1 element and nest it 5 deep in sectioning elements, the document outline leads us to believe that the heading will be a h6, back in the real world the heading is a h1.

But that doesn't mean you shouldn't use multiple h1 tags. It only means you need to be careful in their usage. It does not affect SEO according to John Mueller and Google but it may affect presentation of your page in a browser. You can use this as a helpful instruction from Mozilla.

However, screen readers are a different story and first scan for heading elements and presenting multiple h1 tags may confuse the listener as to where a new section resides in the document structure. See: https://css-tricks.com/document-outline-dilemma/

For these users, the only effect of the Document Outline Algorithm was that some new pages (eagerly adopting the new spec) were presented as flat lists of level-one headings, with no structure at all.