Too often the original article's title just plain sucks, even if the article content is good.
Many authors are good at writing interesting, informative, and useful content, but simply suck at choosing good titles.
I think some of them don't realize that on news aggregation sites like HN, articles often sink or swim by their title. At the very least, HN users will often not bother to look at something with a boring or generic looking title, and never know that the content was worth it.
Of course, if the submitted title was clearly abusive, like some ebay auction titles, using visual garbage like "* * * * L@@K * * * *", then those those submissions should just be deleted and their submitter warned and then banned if they persist.
But changing titles (by their submitters) to something more informative and interesting than the original article title should be allowed and even encouraged, as long as the new title is not misleading.
If HN admins did this themselves, that would be great. But way too often they change the titles to be more boring and generic (which is sometimes closer to the original article title, and sometimes not). This helps no one. They're effectively making HN less interesting, because fewer people will look at articles with boring and generic titles.