I'm talking about the specs in the article (not how browsers interpret errors). So </br> may be interpreted as <br> but is actually a syntax error.
I quoted the HTML5 specification in the VALIDITY section of the article.
I don't see any good reason to use "</br>", but there's some other cases that could be useful, like not requiring spaces between quoted attributes (name1='value1'name2="value2"). I see a parallel with this and the evolution of natural languages: words and syntax that used to be incorrect gradually become accepted as part of the language and attain a normative meaning, because everyone still understands.