Semantic elements and attributes provide a higher level of communication. Communication is pretty much the point of language design. Lay people looking only at how a page displays may never get that additional communication, but machines can. Providing that extra meaning allows machines to translate it for people.
Semantic HTML is important to authors because as John Allsopp has explained,
We need mechanisms in HTML that clearly and unambiguously enable developers to add richer, more meaningful semantics-not pseudo semantics-to their markup. This is perhaps the single most pressing goal for the HTML 5 project.
But it’s not as simple as coming up with a mechanism to create richer semantics in HTML content: there are significant constraints on any solution. Perhaps the biggest one is backward compatibility. The solution can’t break the hundreds of millions of browsing devices in use today, which will continue to be used for years to come. Any solution that isn't backward compatible won’t be widely adopted by developers for fear of excluding readers. It will quickly wither on the vine.
The rationale that HTML5 should have built-in semantics was
successfully used to keep
hidden in the specification. The
Retain several newly-introduced semantic elements, attributes, and
controls change proposal stated,
Semantics are universal. Expressing semantics with native elements promotes Universal Access better than expressing semantics just to users of AT by using ARIA attributes.
The decisions on those issues set the precedent that native HTML semantics is a strong determining factor of what is included in the language.
Please consult the Bridging
aria-describedat documents for the full explanation of why ARIA is
not viable. Shirking off the responsibility of providing long description semantics to ARIA is retrograde. In other words, the premise is entirely backward. ARIA should be used to augment missing native semantics of HTML as necessary, not as an excuse to kill and to replace them.
As the Accessible Rich Internet Applications (ARIA) specification states,
WAI-ARIA is intended to provide missing semantics
HTML is not missing long desciption
semantics. It posesses native, built-in long description semantics
that are conveyed to assistive technologies as
well as other technologies with
longdesc attribute is
unequivocally the most clear, direct, explicit, and
strong semantic with the broadest support base
for providing a long description of an image above and beyond any
other attribute or element.
This undeniable semantic requires no heuristics to interpret which consequently enables precise programmatic determinability to take place.