
I'm not a writer but I'm guessing they do it as part of the world building and to help with the pacing of the relationship of the main couple. Sometimes you can only write so much direct conflict for the main couple. Having someone related have drama allows the relationship to progress for what seems to be a longer time.

making a spin off relies on the fact that the people who read your first work will also pick up the next one. its probably easier to just lengthen the featured comic you already have. and also, the author may not have a whole entire story for the 2nd couple planned out that would call for a whole other comic.
once the 2nd couple gets together it'll probably tune back to the 1st so dw
I'm probably the only one with this opinion but...
I kinda hate it when the manhwa suddenly shifts the story to a different couple under the same title...like it's okay to have another couple weaved in with the main couple storyline, but not completely moving on to a different story.
Why not just make a spin-off series?
I like the 2nd couple too but I want to see the original couple...can't they just close that story and make a new one focusing on that couple instead of piggybacking from the first couple's story?