Any screenwriting class will tell you that your film script needs at least one subplot. They're what's going on when the central characters aren't looking, because audiences like to think that the world doesn't *completely* revolve around the gaggle of dorks that make up the A-plot. Some movies have one subplot, others have multiple, but all are important to making a script function.
But some are more important than others. A subplot is a vital thing to a script's anatomy, and some are so important that their absence would do more than just throw off the structure of a film, their absence would make the film fundamentally not work on a thematic level.
Whether they're B or even C-plots, these subplots saved the movies they were made for, either adding structural complexity that made it more interesting, expanding important characters in ways the A-plot couldn't, or giving the film's theme the necessary meat on the bone.
10. T'Challa's Quest For Revenge - Captain America: Civil War
In terms of the MCU's films, Civil War is definitely the darkest and most tragic entry besides the last few minutes of Infinity War. And even then everyone knew on some level that most of the folks who died in Infinity War were coming back. Civil War, meanwhile, broke up The Avengers, and they stayed broken up for half a decade.
Even when they did come back together it was just for one movie and then they all either died or retired or ran off to hang out with Chris Pratt and his gaggle of emotionally traumatized space weirdos.
So obviously, such a downer of a movie needs at least ONE person to come out better for the experience. Enter the first appearance of the character everyone was hyped for/worried about, Prince T'Challa, The Black Panther.
Black Panther's journey of revenge for the death of his father not only makes every fight scene all the more intense - because on top of Tony and the government chasing him across the globe, Steve's also gotta worry about the world's angriest furry coming in to take a bite out of his best friend - but it also leads to the film ending on a slightly more hopeful note. Having him be the one to actually let go of his anger and his grudges at the moment of truth was a truly powerful moment, and both helped the film connect with audiences as well as cemented T'Challa as the great character he is.