To me, porn games are the successor of softcore sex comedy movies, but better (at least potentially) in many respects. Great sex comedies had strong humor, relatable characters, solid narratives, wacky softcore sexual hijinx, and a clear position that they represented fantasies rather than reality. Revenge of the Nerds (just the first one though) is probably the best example of this in my opinion.
A great porn game, at least to my taste, has all those features but amplified. A relatable protagonist, who you often get to name; occasionally the humor lands (this is probably the part of indie porn games that most often falls flat); they can indeed have an interesting narrative that might even have a point of some kind; sexual hijinx of many kinds that are sometimes fully hardcore, and they pretty clearly warn at the outset that they are indeed fiction (whether this is read or understood well by players is another question).
I would say that some of the best porn games out there, pick your favorites (I like Summertime Saga, Loser, Sexbot, Five of a Kind, Regional Manager, Peasant's Quest, and Become Taxi Driver to name just a few), are quite often in that style. So in essence, I like porn games because, at their best, they are head and shoulders above some of the best baitin' material sex comedy movies have to offer but have a similar feel.
Note: this is not in and of itself intended to denigrate other porn genres. I like porn stories, porn captions, porn comics, PMVs, and regular porn too. But porn games are a whole different masturbation experience. Also, while hardcore porn that tickles my fancy is still great, I feel like the more niche kinds of porn have become more and more softcore of late and thus less my scene.
. And as Pretentious Goblin wrote earlier, both porn literature and porn games do much better job at portraying our nasty, delicious fetishes than the mainstream porn industry.
