Hugo has a tendency to thoroughly describe the setting and characters first, and then tell you what they do there. He actually describes the members of M. Gillenormand’s favorite salon in much the same way – and detail – as the ABC students, even though they disappear from the narrative quickly.
They’re the people Marius grows up around, which has an influence on him, and there’s a nice structural parallel with the people he ends up falling in with as an adult. And yeah, there’s the Titanic comparison, but still…
It’s like taking the time to introduce everyone who appears in the Mos Eisley Cantina. Or maybe Canto Bight, because the casino and the salon are both in the story to say something about society. The Cantina is mainly there to point out Luke’s naivete.
Don’t get me wrong, I remember Tales From the Mos Eisley Cantina being a fun read, but A New Hope didn’t come to dead stop while we learned everyone’s backstory only to never see them again.