As with all fetish stuff, most people get into it as it is originally viewed as taboo growing up. Fat fetish and the like stems from that as well, but with a layer of control to it that makes it appealing compared to other fetish stuff because it generally takes time for you to reap the rewards and in many cases you don't need a plethora of examples to pursue the interest.
This game/virtual novel/dating sim explores that concept as a method of self-discovery through the eyes of the main character on her journey to gain weight, but also with the opportunity to share that interest or spread that interest with other people in her life. This game also explores other fetish stuff as well such as BDSM (as both the dominator and the one being dominated), Vore, and Feederism for a few examples.
The writing is well paced so that the player (for the most part) knows what to do and how to do it, so that they can enjoy the story, images, and character interactions without much effort. It can be enjoyed both as an interactive story and as a game with certain puzzles and aspects being explored, or taking a simple route and exploring it to an ending. This makes it an adventure that may have common elements, but is a truly unique experience on the journey from player to player. You can either relate to main character as an extension of yourself, or perceive her as your own personal feedee to manipulate at will.
This level of interaction brings it ahead of just porn as your personal experience and the steps you take to get to that point feel special and unique. The goal isn't just to get to the fattest picture, the ending or whatever makes you come, but to experience the genuine effort put into the project to convey a story that rides the line between creator and player. People may have simple reasons for enjoying this game, I, for instance, only picked it up because I wanted to look at the plethora of AI images of fat women, but as I played the game and got to see how those images flowed together with the overall story, I found myself having much greater enjoyment than simply image folder fodder.
At its core, it was made with care by the creator who shares this interest/fetish. The art may be made with AI, but it was made with a level of consistency that you generally do not see in most other AI art, meaning that the creator had to carefully work with the generators or edit the images manually to remove extra fingers, bad anatomy, randomized clothing, and other inconsistencies. The game is also still under development, so while there are some typos here and there, it just means that there is room to grow until the final release. This adds to the anticipation of the experience allowing for a growing community of people with similar interests feeling okay with sharing their feelings about things in a conducive environment.
All in all, if you like big, beautiful women who have the penchant for growing larger in a wonderfully crafted story with multiple paths, endings, and experiences, I would highly recommend giving it a try. If you don't enjoy those things, I merely ask that you not cause trouble for those that do enjoy these things. I hope this answers your question.
Cheers!