I actually think a romantic subplot if done properly is the single best way to improve every non-romance story, and even some ACTUAL romance stories. It's cheesy, but it's also an effective way to make the characters seem more human and relatable, brings a good amount of juicy tension, and often gives readers and viewers an additional reason to root for the characters instead of gambling solely on the main character being charismatic enough for the audience to root for.
Take a show like House MD. Gregory House is a know it all asshole. He's sporadic and treats people like shit, and while he's a brilliant doctor, that wouldn't have carried the show for long. The romantic subplot between him and Lisa Cuddy added that "will they, won't they" sexual tension that lots of viewers love, humanized House enough for the audience to see he's a lot more emotionally-driven than his demeanor would have you believe, and episodes where their relationship progresses are some of the most popular episodes of the entire series.
Now take another series, a series that's practically nothing like House MD... let's use Adventure Time. It's a wacky kid's cartoon full of random events that often turn nonsensical. Yes, there is a great amount of cleverly written lore in the episodes, but undoubtedly a large, if not the majority, portion of the fanbase watch it for the romantic subplots such as Finn's crush on Princess Bubblegum, Princess Bubblegum and Marceline the Vampire's mysterious lesbian-vibe relationship, Finn's relationship with Flame Princess, etc. for pretty much the same reasons that viewers liked the romantic subplot in House MD.
Not every story needs a Romantic subplot. Some stories do just fine without them. And even if they do have one, the romantic subplot doesn't actually have to resolve itself into a relationship between the characters. But it's a very versatile weapon that can definitely give stories that extra appeal.