I think this is to build up the foundations of the aspect of cosplaying and how many people start and go with it with different perspectives. Can't just have fluff romance and cuddles every chapter or else what's there to the story than just that? and I, myself, think that this flow makes the romance parts even more better ^^ and to look forward to
This is fulfilling the slice-of-life aspect of the manga, and we all know slice-of-life from novel writing, manga drawing, animation, corporate work, restaurant service and other aspects of life.
Sure, it can go with just them romancing and slap in the cosplay part to it, but that'd just make it seem like a convenient theme.
It's fine to have one or two characters appear for informative world building; like photography and cosplay production from the first two sisters.