For me, it definitely spun out from a real-life fear that I had of gynecological examinations and a deep-seated modesty. I did not lose my virginity until I was 18 and six months later, I got married and avoided the gyn altogether for a couple of years into the marriage. The first time that I went to the doctor's office that even remotely required that I uncover my private areas, I had been complaining of constipation and was requesting The Pill. There was something about the MD's approach that was very off-putting and I didn't feel comfortable at all; she was actually condescending to me, which I did not appreciate well and I did not allow her to proceed.
Looking back on things, I was of course very immature, but there was a lot about that office visit that did not work for me. So naturally, I avoided returning to the doctor - any doctor - for about another year or so. Then, when I started to have reactions to something in the condoms we were using, whether it was the latex or the lube or the combination thereof, it became clear that I was going to have to "woman" up and go to the gynecologist for real. Before I did, though, I went online to "research."
At the time, I did not even realize that medfet was a thing. I kinda stumbled upon it when I was trying to learn everything I could about it. And, quite honestly, if it weren't for some of the medfet exams I read about online (there used to be a really good script on Doublemoon Paradise that was very intriguing to me) that relaxed me enough to consider going in to the actual gyn. It built from there. That said, anytime that I have been on the exam table, I have never actually been turned on by anything that has actually happened in the doctor's office. Curious, yes - turned on, no. In fact, I've had a couple of procedures through the years that I would rather not remember.
As for my medfet, I have not delved much into the actual world beyond having been an on and off again member here. I am a nurse in real life and one job that I used to have gave me access to some supplies that my husband and I tried - but he is not turned on by this like I am. I enjoy being catheterized very much. I have never been administered an enema or a suppository and it was so uncomfortable when I tried it myself I do not see the point in including that in my "dream exam" (though, of course, it is still in that scenario). For the most part, I watch an exam video from time to time when I am by myself and if it's a good exam which may or may not be vanilla+, I enjoy the effects that the visual of it has on me. And of course, I have a scenario in my head I would like to experience. Then again, if I was actually approached with the opportunity to do so IRL, sans my husband (because, since he gets nothing out of it, he is not part of the fantasy) - highly unlikely - would I actually participate? I would like to, but I don't know what I would say if I was faced with it in reality.
So to answer the question in a very round about way, it was a way to deal with my fears. And now, it's something that I enjoy privately.