OnlyUsedForNever has given a good explanation already. point 1. is probably the de facto main reason for galleries.
however i would like to expand on point 3. :
if you have read my review of this game you might have seen that i listed scenes the game has, based on composition. for me an important part are for example moresomes. the most obvious ones are ffm (2 female 1 male), mmf (2 male 1 female), gangbangs, and reverse gangbangs. because for me if those scenes are part of the game, they tell me a lot about how the story handles jealousy.
this game is a harem game and you may have noticed that usually harem games are about 1 guy and 1 guy only, while there are a myriad of girls who play bigger or smaller roles in said harem. to quite a few people these haremgirls cannot be touched, interact or even looked at by other males, as they clearly pose a threat to the harem. sometimes even the haremgirls aren't allowed to sexually interact with each other.
or what i recently noticed, exhibitionistic girls who do dances or stream themselves, to give them a "slutty" vibe while still being virgins or at least extremely inexperienced, have to stop their activities once they join the harem.
this extreme jealousy is something that immediately makes a game boring for me.
the other way around can also be the case. you have a sharing/cuck game where the male protagonist almost never or actually never gets to have sex with their own partner/spouse, while she gets guy after guy with barely any consistency.
this too makes games incredibly boring for me, as i rather would have gone for a female protagonist and wouldn't care for the partner/spouse at all instead!
so while viewing a gallery of a game i haven't played, i can see if a harem game has sharing to extend i like (even exhibitionism in front of male onlookers suffices most of the time), or if a sharing game has a male protagonist who gets his fair share of girls as well! because i want both. i want guys who can have fun with multiple girls and girls who can have fun with multiple guys. extra bonus points if it isn't cheating in any way. therefore a game like AlwaysFan with both a main male character and a main female character, who both share in sex scenes with an additional partner both either male or female, or both male and female together (partner swap) are the games i am looking for.
but try to find that out by looking at the game's summary and tags! ntr is too broadly used, swinging might just be one-way and harem doesn't say anything about how jealous the protagonist is.
that's why sometimes i give the gallery a look to decide if the game is something for me.