The thing about "side" couples is that they are multifunctional: they serve as a foil to the main characters' hijinks, serve as a break from usual drama and/or give us different drama to think about, introduce new stories without having to jump to another manga and lets us know more about the life of the seconday characters that we may end up linking, wanting to know more about and avoids having them become one-time characters.
I mean, look at the main couple: all misunderstandings got resolved fairly quickly, they get over their emotional issues rather fast, spend goot time together learning more about the other and overall are a healthy couple that progresses at a good pace; if we didn't have side couples and side stories then the manga would've ended in the second volume and not only we'd wouldn't get to see more about the main couple but we'd lose our minds screaming at them for the mistakes they'd have to make in order to keep the story interesting.