One of the most obvious reasons to have a permanent transformation is that describing the inverse of many types of transformations is aesthetically repellent to many readers. Some people very much enjoy reading, say, a weight oss transformation (especially as part of a trait swapping story) while others won't want to read about that in any circumstances. Same goes with many gender change or age related changes - a reversal might be off-putting.
Another reason why a permanent transformation is that it makes the change itself more meaningful and full of tension. Sometimes the characters don't know the change is irreversible, but the readers do. Other times, I have a character willingly choose to transform (or to complete the process) knowing that they can never go back. In earlier posts, I've made the case that nearly all stories are transformation stories, it's just that the transformations are usually about character development and personal growth. In "traditional" narratives, a transformation can't be undone in the same way one cannot undo growing up (unless, of course, we're in an age regression story...) A transformation is often a gateway to a new life, one to be lived full time, and one that entails leaving one's old life behind, forever.
Having said all that, there are a few cases in which a "temporary" transformation may be appropriate in my view. I've written a few, particularly some of my old AP stories in which the transformation was more oriented around a magical disguise, rather than the character themselves changing. Another way temporary transformations work is in the world of some super heroes (think of a 'Hulk'-like character changing based on their emotions) or for some monster characters like werewolves (think Destiny of the Huntress.)
What most of these temporary change stories have in common is that the change itself is not the actual transformation. The transformation is going from a normal person to someone that has the ability to have changes temporary changes. It's a subtle distinction, but it's one worth noting. And, in that sense, it's often permanent.
I was reading a gender change story recently, and the change was both temporary, and non-permanent. That is, the character changed into a woman and then back into a man, and had no intention of changing again. I felt dissatisfied with this story. I don't like admitting that I prefer stories to follow a certain pattern for formula, but in this case, I felt the story was saying that the change wasn't going to be an important facet of the person's life. If that was the case, why write the story about the change at all? Just to describe a full weekend?
What about you? Do you prefer permanent changes, or ones that can be reversed for various reasons? And why do you think you like what you like?