Does the answer really differ from the enjoyment took from none "extreme" fetishes ?
I'm sure that there's tons of people wondering what people with foot fetish enjoy about it. Same for femdom (without the BDSM part that can goes to the extreme spectrum of fetishes) when you're more on the dominant side, or at the opposite maldedom when you're a submissive guy. Any fetish have something weird for those who don't have it, and is difficult to understand because of this.
The fact is that enjoyment isn't something totally rational, especially sexual enjoyment related from fetishism. Even for something as simple as kissing, there's those who can't have sex without doing it, and those who totally dislike it.
But there's also another factor to take in count, this is all fantasy. Therefore there's people who would enjoy to see this or that fetish in games, but wouldn't enjoy it in real life ; and there's those who accept the fetish because it goes with the story, but don't effectively enjoying it.
By example, it's frequent in corruption games to have pissing scenes (what is different to the uro fetish by itself), but isn't it fitting the genre ? The girls is so corrupted that, if asked to open widely her legs when taking a piss, she'll do it.