Why an editor to mark the page with language annotation? This does not affect the visual outcome. Why this is even possible via VE?

@Magioladitis: This feature provides good HTML markup and prevents problems when different scripts are combined. It may be helpful to some readers if they are reading in English and a Spanish word or sentence is present, but it is essential when reading or writing in a left-to-right script and then switching to a right-to-left script.

The HTML markup has the same effect as the template {{lang-ar}} on the English Wikipedia, but with the additional benefits that it works at every wiki (regardless of the presence or absence of local templates) and doesn't require transcluding a template.

That is why it is possible. But why would someone do it? It's possible that some of them believe this is a built-in machine translation tool, rather than a way to change scripts. Providing a clearer description is the subject of T117460: Clarify the purpose of the Language character styling option.

Any interest in preventing a user from adding a <span> tag that matches the language and LTR direction of that wiki? Something like this might have helped prevent these 113 cases of English pages being marked as <span lang="en" dir="ltr"> by newish Visual Editor users.

https://en.wikipedia.org/wiki/Wikipedia:AutoWikiBrowser/Tasks#Remove_span_lang%3Den_dir%3Dltr_from_113_articles

Preventing seems like a bad idea. However perhaps having a warning "this is already the language for the document, are you sure you want to mark this specific part as "content language" " or something might be an opportunity ?

Most of these look accidental: https://en.wikipedia.org/w/index.php?search=insource%3A%2F%5C%3Cspan+lang%3D%5C%22en%5C%22%2F&title=Special:Search&profile=advanced&fulltext=1&ns0=1