i remember watching a video of a naked man eating his shit out of a plate with a flaccid cock, gagging repeatedly with every bite. i still don't know why he did that.
That is the extreme side of the fetish, just like any other fetish there's a point where people can't control themselves and go all out on something, most fetishes can have their extreme side. Although, from what you're describing, it sounds like he wasn't enjoying it which says it might've been linked with another fetish like sadistic humiliation (for the viewer to enjoy) or maybe domination or something.
Not everyone into scat does stuff like that, a lot are just into observing a cute/beautiful girl poop or pee as it's something the girl would rather not be seen doing by anyone because they find it very embarrassing. They don't get off to the poop/pee by itself but rather the act of pooping/peeing is what gets them off, a lot wont even touch the stuff let alone eat it. For instance, to men, most girls are seen as adorable, cute, soft, clean, healthy, elegant, beautiful etc, so the idea of them doing something so private and so dirty is almost like a fairy tale fantasy to them, and so witnessing them do it can be such a big turn on knowing they actually have a natural but secret dirty side.
As
DocRipper said a page back, you can say scat is associated with sub-fetishes such as humiliation, voyeurism, exhibition etc. Pooping is a very private moment for most people, so the thrill comes from seeing someone do something that is rarely (if ever) seen by anyone due to embarrassment and privacy, most people don't want another person knowing or imagining that they poop, so that could play a role as well. Omorashi (aka pee/poop desperation) can also play a role too, which is linked with humiliation.
So yeah, there are lots of ways someone can view this fetish outside of just boiling it down to "They're into scat, so they just like to eat poop and rub it on themselves". The fetish is very old, maybe even primal, some parts of it can't be explained, just like a lot of other fetishes really, but I think the more people aware of this information, the better. Ignorance and misinformation be bad etc etc, yadda yadda...