Can't speak for anyone else of course, but for me specifically, what you're asking is a multi-faceted question that I'll try to seriously answer (and treat as if it isn't a likely shitpost, you never know for sure);
In regards to fat women in general, I like that they're big, soft, warm and heavy, and that bits of them jiggle and wobble in motion, basically the same principle as big tits but applied to the rest of the body and magnified significantly as they get bigger. The side aspects of the needed financial/resource stability required for someone to get seriously-fat and the physical pleasure of being that obese for those who are into it are also nice bonuses to add into the mix. Plus, it's kind of a badge-of-honor in a weird way to know that you're able to keep your partner fat, perpetually-full and comfortable, at least as someone who was raised in a pretty unstable, chaotic and stressful household growing up.
For feeding specifically, the act of feeding someone you care for romantically can be quite sweet, even if the thought of their BMI going through the roof as a direct result doesn't get you horny, though it naturally doesn't hurt any if so. It's a close and intimate thing for lovers to do, and it just gets even better when one of them is a fatty with an immense appetite as well, especially when the direct result of said feeding is that there's more sensitive softness to grab hold of every time things get hot and heavy.
With the whole "giant helpless immobile mound of flab" thing, that's a concept very commonly seen in fat fetish fantasy on the far extreme of possibilities, which falls apart conceptually the exact moment you think about how the hell someone like that would be able to do much of anything normal in reality, up to and especially including using the bathroom. Even ignoring those obvious issues it's not for me, I prefer things to stay within the realm of "still able to walk unassisted" sizes, if for no morally-better reason than because I want to see all that flab jiggle as much as possible. There's people out there who genuinely want to try and fatten up their actual IRL partners to immobility, but considering the human body doesn't take well to that health-wise to say the least, alongside the aforementioned obvious logistical issues of being too heavy to stand/walk or do basic self-care, I shouldn't have to point out why that's a horrendous idea.
And lastly, given both parties in the image are clearly enjoying what's going on, I'll give my two cents on the topic and say that I find consensual feeding and gaining vastly superior to forced, even if the latter can still be decently-arousing if written well in stories. Something about someone being all-for-it and enjoying being fat/being fed/gaining weight makes it so much hotter to me than if it's secretive or forced on them. Also, should go without saying, but it absolutely should be consensual and out-in-the-open for any IRL relationship too, no good and healthy relationship was ever built off of lies and deceit no matter what kinks were involved.
Anyways, thanks to coming to my TED talk on fat chicks and why they're the best shit ever. If you'd like more info, just look around literally anywhere on here, can't shake a stick without it getting stuck in a fat roll