An important and interesting question!

I can think of at least two reasons it makes sense.

• assuming this is a manual translation and the author is multi-lingual that would their own words so it would be different from automatic translation, and that, let's be honest, still sucks sometimes :))) Especially when it comes to full articles, gender when applying to adjectives, etc. (that's often true when I'm reading something auto-translated into my language)

• on at least two, completely separate occasions, I've read comments by large stake holders here they give bigger upvotes by counting the words in a post. As a visual guy who prefers images, this makes me a big sad but it is what it is after all. Still, I bet some people include translations just for that purpose - to increase the word count. Lorem ipsum, hehehe...

From a technical perspective, it doesn't make much sense, I occasionally use the "Translate..." feature that's part of the Chromium-based browsers. One click away, no need to copy/paste and once activated it works on the same web site until the tab/browser is closed, so it's convenient :)