They worked so hard to research and create a 2000 word article that accurately reflects the world (in accordance to their editorial guidelines) using their good name - and then let fReEtHiNkErChAd and 2020BloodInTheStreets chime in at the end.
It would be OK if they strictly moderated the comments, but few sites seem to do that.
I'm not sure which news sites you read but most of the ones I've seen count only as journalism in a very vague and abstract sense. The information is thin, the writing poor, and major important questions go unanswered and ignored. If there is indeed any point to following the news on a daily basis (and there is good evidence to suggest there is not), the quality of most new sources is atrocious enough that none of it is worth my time.
(The exception here is the occasional piece of very well done investigative journalism.)
But, in regard to comments, they do it for the same reason that Hacker News or any other site has a comments section: community engagement. This is not necessary a bad thing but it comes with the responsibility to _moderate_ the resulting community and almost every site drops the ball on this.
I'm pretty sure a big part of the reason the Guardian stopped opening comments on most stories was the cost of 'moderation', except the reason their moderation costs were so high is that their moderators would routinely plough through comments by hand to remove people pointing out mistakes or absurd statements by the journalists, and as the Guardian changed over the years the articles became ever more extremist, so that started to be most comments. That is, they defined making their own staff look bad as abuse (regardless of whether it was written in vitriolic style).
Even now if you go read one of the few open threads they have, you can find commenters pointing out logical or factual problems with the column.
Authors of articles are generally more informed than their pieces suggest, but their editors step in and make the story "pop" for readers in order to generate clicks. This often involves removing nuance. I believe that most major news organizations make an honest effort to be factual, but there's a lot of room within the facts to be misleading.
A commentator that is an expert in the field is not constrained by an editor, and can in a few paragraphs give a more realistic and accurate assessment of the phenomenon.
When I read articles about things in which I'm not knowledgeable, I don't trust the conclusions until I see high quality comments(or tweets) that confirm the thesis of the article.
Sometimes, to some degree, and sometimes not. The level and consistency of accuracy varies per organization, per individual reporter, per story, and per person doing the judgement of "accuracy".
I find this laughable. Many recent articles I've seen are designed to get clicks and have nothing to do with good journalism or research beyond what someone can get from behind a keyboard.