Ok, I'll do my best to explain it to you. Basically, a ridiculous amount of fetishes are based on curves, because the average human male is supposed to like them from an evolutionary standpoint. Things like BBW fetishes, big breasts, big butts: they all involve curves. It's easy to see how inflation ties in: depending on the artist, you can get anything from just slight expansion of curves to flat-out spheres. But beyond that, there are other aspects that increase the attractiveness to those with the fetish, such as imagining how it would feel to touch an inflated girl or how it would feel to be inflated. Personally, I enjoy the look and feeling of "pressure" on the victim, like they're overfilled and about to pop, but I don't usually like popping. I also like the elements of domination present in the fetish, because that appeals to my sadistic side. Finally, I'm just a really big fan of soft/warm things in general.
As for how they would feel to touch/fuck, I'd probably imagine something close to a bounce house/other large inflatable object. Besides the softness, fucking them would be cool because everything would be bigger, and their sex organs would probably be more sensitive due to being expanded.
As for methods of inflation, it depends from person to person. Some people like it more realistically grounded, while others like to just fantasize about all sorts of weird shit. If you hadn't guessed, I tend to fall into the latter category.