I have to say this before I start this thread, I have no problem with how you as a writer create stories, as creative freedom is one of the most important things for writers and I have no problem with you having the genderbending tag or anything that relates to it. This is me ranting about the story-based purpose for the genderbending tag, and why I personally don't like it.
It honestly makes no sense to me when someone decides to genderbend a character for no reason other than to make said character the opposite gender usually for 'Lesbian Hawt' or 'Gay Hawt'.
From what I have seen browsing multiple books regarding this tag, it doesn't add to the story and the gender-bent person adapts to their new life as if they were said gender, so -- from the stories I have read -- the story doesn't even take on the whole woman trapped in a man's body thing or vice versa. The only story I have seen take an interesting take at the whole genderbending thing is 'Saga of Tanya the Evil'.
So, my question is, "Outside of fulfilling a sexual fantasy, can you find a reason for genderbending?" Because honestly, I can't.
Again, I don't have anything against those that use the tag or enjoy the tag, this was me ranting about how I believe the genderbend tag provides little to no purpose to the story.
There are several, actually. *puts on professor hat.*
Gender-benders are a concept that has gained in some popularity and appeal, as seen by the relative success of several of the titles that have done it. The most popular of which is the very trail-blazer to the concept, Ranma 1/2.
I will start from the most powerful and likely reasons for gender bender titles, and go on to what I consider the least important.
1. Easy way to justify a "chick with dick" protagonist. That is, someone tries to write a "strong female character" and instead winds up writing a female character that just comes off as too awkwardly male. Gender bending a male into a female manages to both make the male behavioral traits more natural seeming and less forced, while also justifying the fact that she has those traits. Of course, this kinda discredits the "strong female" part of it a little since she technically has the mind of a male to start with, but in terms of character it does work together pretty smoothly.
2. To go with the above, it allows you to write a strong female character who is less threatening and off-putting to some segments of the reader audience.
3. The trans wish-fulfillment others have commented on. I personally dislike this approach. It also often winds up completely trashing the strong female approach with the observations you listed in the OP with how the gender-bent character winds up just acting like their new gender anyway. When you see the supposedly originally male character acting like the female they have become, you know it's just trans wish-fulfillment and nothing else. That's the best and easiest way to identify when that's what's going on.
(I may sound really down on this. I have nothing against the trans part of it, and I support that. What I am more against is the 'wish fulfillment' part of it. It just feels like a squandering of the opportunities you get with gender-bent characters.)
4. Exploration of gender roles. This is what some of the earliest entries in the gender-bender genre did, back when it mostly appeared in Japanese Manga in the wake of the massive success that was Ranma 1/2. This is a little bit of a higher level of writing, and a little over my head to even approach. So, I will not comment further on it.
5. Various criticized approaches, such as it being for some sexual fetish or out of bandwagoning. There are varioius sexual fetishes. And, judging from the number of gender-benders out there that tend to go a sexual route, I think there's a lot of this out there. More of it is the band-wagoning though. They see the success of the titles that have the more noble views of the 4 entries listed above, but they do not genuinely hold those views themselves. Therefore, they just look at the sexual aspect of it. As such, bandwagoning and sexual degeneracy tend to go hand-in-hand. In fact, the sexual degeneracy is often the sign you look for in order to identify whether or not it's just for bandwagoning.
6. Realism in reincarnation. Some people, such as myself, might find it an interesting prospect to explore the idea of reincarnation a little more. Namely, it actually is kind of illogical to think that you would always be reincarnated as your same gender. I mean, I suppose it makes sense if your reincarnation is facilitated by a god. If it's the natural occurrence route, though, then it is more sensible to think you have a 50% chance of winding up the opposite gender as well.
I have a gender-bender series myself, and this was a rather large part of my motivation. I decided to take an angle of the first 2 items on the list as well in order to better justify it. But, if I'm being honest, this is the real reason that I had in my mind. It was really nothing more than an attempt at a gimicky subversion of the tropes on reincarnation. (My main cast of 4 characters contains 1 example of each of the 4 possible previous and current life gender combinations. (male reincarnates as male, male reincarnates as female, female reincarnates as male, female reincarnates as female))