As a fan of this couple, I enjoy these scenes just for their existence, I love this romantic stuff, I watch all of it with stars in my eyes!
But the thing that I don’t see talked about all that often (and I don’t mention it often enough myself, either!) is that it’s not just there for the audience to enjoy (aside from everything that ever happens in any story is for the audience to enjoy) but that it’s there because USAGI EATS THIS KIND OF THING UP WITH A SPOON LIKE IT’S FUCKING ICE CREAM.
She loves this kind of thing! She loves the formal gowns and tuxedos, she loves pretty jewelry, she loves fancy parties and dancing, she loves the cool and reserved prince type, she loves that he’s actually entirely warm-hearted underneath, she loves being treated like a princess, she fucking LOVES THE COOL PRINCE TYPE.
This right here is what gets Usagi’s gears going!
When you look at the people she’s most attracted to (though, you can make cases for her Sailor Senshi, but they tend not to be as consistent, except maybe Minako), you have… Mamoru, Haruka, and maybe Seiya in the manga. And all three of these people are very “cool prince” types of characters, who make these sort of sweeping gestures, who pull her close and physically shelter her (which is more about emotional support than actual physical protection), who treat her like a princess. Usagi has attractions outside this type of character, but I’ve always seen her as very definitely being the most weak to this kind of person.
So, this romantic formal ballroom type of party isn’t just for the audience reading/watching this story, it’s for Usagi herself as well, because this is what she goes for, this is the kind of thing she loves, this is why she’s so attracted to Mamoru, because this is the kind of person he is much of the time. This isn’t all that’s there to EITHER of them, but it’s definitely one of those things that Usagi loves, much like some people are attracted to a person with a sense of humor or who is really patient or what have you. This is Usagi’s thing.