Benneq is a popular character of CoffeeSoda's that appeals to a lot of folks, and the description of the game at the top is literally "fuck Benneq in this experimental sex game...", so I'm not sure how you missed that, lol. :p It also makes the point, somewhere (game's intro / first few text lines?), that this is about him going to the cafe to get banged by a bunch of dudes (and his wife as the finale) as part of some little challenge.
At the beginning of the chart, it says "Featuring" - implying that the following characters are in-addition-to the "main" character, Benneq. Much like when a popular artist features a guest singer, etc., in their song. In this case, they're just the "guest" dicks. If it would have benefited to just be that much more explicit, I suppose it could have been written as "Featuring the dicks of:..."
The character descriptions are just a little bit of extra flavor and personality, really - along with being inserts of some folks' sonas. (Were they backers of the game, perhaps?) It could have just been a game featuring Benneq and a bunch of interesting-looking, disembodied dicks that you fuck him with, only - sure. Then it would have just been that much less of a game/story/context for the whole thing. In that case, story-wise, it might as well be "fuck Benneq with a bunch of interesting toys" instead of the assertion that there's whole characters behind each of the dicks that are fucking him.
With it, though, there's a little bit of dialogue and personality behind the dick - there's a whole person behind it (and it's just disembodied for the sake of the game and/or smut). The goat-demon causes a pentagram to show up when he cums and teases about avoiding holy water for a while, the shapeshifter grows his dick during his turn, and one of them just has two dicks because reasons. It gives a little more flavor and context to it, that's all - an aid to fill-in the experience/story for the user between the lines of what's displayed.
All that said, it would indeed be fun to see other character choices for Benneq's role. That's totally a valid desire. c: