If you have the time, could you please explain on why starco is important to the main plot in the show? How does it affect it? Would it be bad if starco didn't happen, if so why? How does starco help for character development? Or just any info on why starco is important and not "unnecessary drama"? (Me and my brother had a serious argument about this and I need your help to find better points haha, he's tad biased.) Sorry for long ask. Thank you though!

Just a handful of really obvious examples for how Starco directly affects the plot, even though it’s just a small part of it, and the way it affects it indirectly are still way more important:

S1 Finale: Star endangers Marco’s and her own life for the sake of a sandwich and some easy fun.

S1 Finale, little later in the episode: Star understands that Marco’s life is way more important than her wand, the symbol of her nonstop fun having. The wand gets cleaved, losing something, but she gains maturity, and a stronger friendship with Marco: the safety of loved ones, be it from physical danger or emotional, is more important than the simple research for “hedonistic” fun.

S2 Premiere: Star is quick to give up in her attempts to dip down, she’s a “skimmer”, she runs away from problems.

S2, several episodes: Marco is repeatedly shown to be part of Star’s impulse control, helping her to focus on important things and accept consequences, as unpleasant as they might be

S2 Finale: Star accepts that running away from problems is as useless as running away from her feelings for Marco

S3 Premiere: having matured over the course of a whole season Star is not a skimmer anymore, and not even dying twice is enough to deter her from saving the day and blasting Toffee in the face.

These are just the biggest examples, overarching entire seasons, but there are several others, often more specific. Like, Marco was the first one to help Star see that not all monsters are evil, in Lobster Claws and Mewnipendence Day, and this is now one of the central themes in the show, and the recent season Marco saved Star’s life twice during the portals arc, first from a harpoon in Night Life, then from magic oblivion in Deep Dive. But really, Starco affects most of the story, because Star and Marco’s relationship is one of the core themes of the show, and S3 inextricably wove together “friendship” and “love” (officially in Booth Buddies, but it’s something that was present from Lava Lake Beach already, or even Scent Of A Hoodie), so saying that Starco is unnecessary drama would be like saying that Star and Marco interacting is unnecessary drama - obviously one can have their own subjective tastes and not like what the season or the show is doing, the amount of drama, the way it get handled… subjectivity is a whole other deal.

Also it should be considered that this is not real life, it’s a TV show. Some drama is needed, or things would be boringly direct, with no twists or build up. And since Star and Marco are the protagonists and the show has quite the focus on their love life it’d be absurd for their relationships, both with each other and other people, not to play an important role in the series.

And let me use Marco as an example for why Starco is useful to the character development, since it makes for a more subtle and interesting case than Star: for all of S1 and S2 Marco has been seen as a moderately mature kid who gave really good advices to people - Star, Tom, himself in Sleepover. But the moment he began getting faced with things that were slightly bigger than what he was used to, the moment he was forced to question the preconceptions that he had built for himself over the years, he failed. Star confessed to him and moved away from Earth, and he didn’t follow (I mean he did but when the danger was over he got back home). he didn’t really want anymore the kind of life Earth could give him, as evidenced by his obsession with the cape, but he wasn’t just ready yet to accept that he stopped wanting the life Jackie symbolized to him the moment Star got into his life. When he did, he started to grow, now having to take big decisions from himself - from moving to Mewni to becoming Star’s squire to accepting that he liked Star and that there was nothing he could do about it. “Starco” allowed Marco to stop being a kid who just coasted in life, and brought him to get out of his shell on his own, instead of being dragged around by Star, accepting that even the best plans can “all fall apart” (the song that plays in Lava Lake Beach), and learning how to work with that.

(by the way, in Star’s case I’d have talked about how the journey to accept her feelings for Marco in S2 overlapped with the journey to accept that problems don’t go away if you just ignore them long enough, leading to her facing Toffee head on and accepting her duties as a princess)