I have a tendency, when writing fan fiction, to take a fairly obscure character form canon and flesh them out, in essence creating a character where there was merely a placeholder. But as much as I create a back story, and use that obscure character in ways the original creator would never have anticipated, I always have it in the back of my head that this isn’t my character. It’s my perception of (or perhaps aspiration for) the original author’s work.
I think @Desiderius Price has a point in saying this is a step towards being able to write original characters for someone who hasn’t taken that leap before, much in the way writing fan fiction in general teaches the necessity of plot, and character arcs, and so on.
Now, from the moderation standpoint, many readers absolutely hate genderbent characters, or characters that are seriously out of character, which is why those tags do matter. Other fandoms make a cottage industry from swapping genders of main characters. Either way, as long as there’s a proper tag, readers are forewarned (if they stop to read the tags).