I frequently write Genderbender stories - roughly half of my stories are GB - and for me it's part of a broader theme exploring physical and psychological transformations and what makes us unique as people. Also, I sometimes just write fun smut. Often a bit of both, if I'm being honest. But, really, this is a theme that many other story tropes tap into. Look at all of the stories that are 'Weak to Strong', 'Strong to Stronger', 'Cultivation', etc. Look at virtually every LitRPG story out there. In these stories, we start with a central character who's essentially unmolded clay, transformed into something greater across the progression of the story. In any decent bildungsroman (coming of age) story, we have a young character developing and coming to realize who they are. Many of these same qualities are embedded in Genderbender stories, as are many aspects of coming to terms with and understanding aspects of your psychological and/or sexual identity. Obviously, there are quite a few stories out there about OMG I'm a boy/girl now LOL!, but if you look at the most ham-fisted examples of any genre, you're going to be disappointed. At its best, the GB theme/genre can explore characters and their development in deep and satisfying ways that are only rarely seen in other genres. And, when done well, the author can offer up a good damn story to boot.