Gender Swapping in Fanfic

Hope I’m not offending anyone or saying something stupid.
But am I the only one who doesn’t get why in fanfics people do gender swaps. Like a lot of the time when I have a m/m pairing I’m reading, I always find stories where either one of them has female genitals or they out right just make one of them a woman.
I don’t get how you can ship two male characters together but feel the need to make one of them female in the pairing. At that point just write a reader story with y/n.