The Marvel Cinematic Universe has been one of the biggest movie moneymakers of all time, pretty much since its existence. Since starting with Iron Man, it has expanded to include hundreds of Marvel Comics characters and dozens of storylines, across many movies and television shows. As will happen when a universe has so many stories going on, some of them have been hits and some have been misses. Among the hits though, there is a subtle underlying theme that has done wonders for the depth and realism of the entire franchise.
The series of course revolves around the Avengers and other Marvel superheroes. Many takes on the superhero genre put these characters on a pedestal. Everything they do is for the good of the world, and the only one who gets defeated is the bad guy that they're fighting. That seems pretty idealistic, and uncomplicated. When the MCU decided to ignore that narrative and pursue a storyline where it wasn't so black and white, was when the franchise showed it was actually worth getting invested in.
After the first big Avengers movie, 2012's The Avengers, the MCU starts to take on a storyline that involves dealing with the fallout of the big battle. While it wasn't something that had never been done before or never discussed, it was still kind of refreshing and unexpected. It took this huge movie with a massive battle that had a lot of visible destruction to New York City, but ultimately had a "happy ending", and added a sense of realism. It seemed more like superheroes were in a world people could relate to, and not so much like this took place in a fantasy realm where they suffered no repercussions.
Following the events of The Avengers, New York City was kind of in ruins. As the destruction of the city was played out right on the screen in front of viewers, it would have been such a plot hole had they chosen to ignore that. Luckily, they didn't. They addressed it, and America and the world were trying to recover from it. The characters involved were also struggling from it and trying to recover. It was shown in films like Iron Man 3, the first film following The Avengers, and was a theme continued throughout the series. The superheroes themselves were dealing with the trauma of what they were involved in, and the terror their battles caused.
If the MCU had decided to go with a squeaky clean image where all of its heroes were always in the right all the time, it's questionable whether its power would have lasted this long. While it would have certainly fit in with the very clean and pristine filmmaking seen in the films, as well as the Disney image, it would have been a whole lot less engaging. Films like Captain America: Civil War and also to an extent its predecessor, Captain America: The Winter Soldier likely wouldn't have come to be. Some of the most standout moments of the franchise, wouldn't have made sense. The individual characters as well, like Tony Stark/Iron Man, Steve Rogers/Captain America, and Bruce Banner/Hulk wouldn't have had the same development that made them fan favorites.
Using the superheroes to address real-life issues like terrorism, destruction, and trauma grounded the franchise in reality. Yes, it's set in a world with a lot of fantasy elements like other worlds, aliens, mutants, and super-advanced technologies. However, instead of only indulging in those elements they've chosen to make the common themes ones that people can really relate to, and they seem realistic. Had they chosen to ignore that aspect of the story, where not everyone is in support of the heroes and people lose their lives because of the destruction caused by these massive battles, it would lose that sense of realism. Taking that away would have caused the franchise to lack a lot of the complexity, maturity, and depth that it currently has.
It could absolutely be argued that a lot people watch movies like those in the MCU for a sense of escapism. That a lot of people aren't looking to see something realistic in these films, and want the fantasy to be indulged in. There's certainly a case for that, and the series still has a good amount of that escapism and fantasy for those people. To hit the mark with most people though, they needed to include some motive and some conflict that had real effects and follow through. People who appreciate good filmmaking and good writing are seeking plots that follow through to the end, and people as a whole are intrigued by things that seem realistic to their world.
The MCU is always growing and developing, and as that happens they seem to tackle more and more heavy topics. The latest batch of projects seems to be heavily focused on grief and trauma, which is in a way the continuation of the post-New York City battle destruction storyline. Introducing this element to the series added a level of seriousness and reality that wasn't there before, and it led to so many of the best parts of the franchise. Without it, the MCU would be so different to how the world currently knows it, and probably not in a good way.