Somebody with an eye for film production costs had the genius idea to build a shared universe among the series of Marvel films. Each movie took place in the same story-sphere as the rest.
Not only did this create a "feel" loyal to the movies' comic book storytelling roots where any character from another storyline could show up at a given moment, winning hearts and minds of discerning fans, it also lessened genre fatigue.
This structure is an audience invitation to learn more.
The Avengers changed serialized storytelling for the modern age.
Here's the open appeal of a shared universe: say you don't much like Captain America or Thor. However, you liked Iron Man. Well, good news, Iron Man is going to show up in this Avengers movie. While you're there hanging out with Iron Man, as intended, you might accidentally end up having some fun hanging out with some of the other characters.
That's where the invitation kicks in. It's a gentle nudge to go learn about the other characters out there kicking around in the same universe that you might not have found too appealing before.
Traditionalist storytellers might call this a cheap trick, but I'll once again summon the craggly gothic sprit of Nobel Laureate William Faulkner, who literally created a Yoknapatawpha County shared universe, which contained The Sound and the Fury, As I Lay Dying, and Absalom, Absalom!
Faulkner, like Stan Lee (I'm so sorry), held all these stories in his head. Face it, if you've written a lot of different stories, there's been a moment when you've tipped back in your chair and wondered in a quiet personal moment, "I wonder if this character I'm currently writing is taller or shorter than a character I wrote two years ago."
Co-existing is a natural state for characters seemingly in separate universes. Your brain already designs them to appear next to one another. Seizing that opportunity is not only more consistent storytelling, it's a great marketing opportunity.
Noticing the writing on the wall, you can spot these potential shared universe ideas show up in a lot of Ghost Little's work, not be accident.
Evidenced by Batman and Wonder Woman showing up in Superman's movie, it does not solve all your problems. A shared universe can be mishandled if treated flippantly.
Readers and viewers value consistency and good storytelling above all else. If you insult the audience's time or intelligence, some ferocious mechanisms activate in their bodies, sending them howling. They have moons to address and plenty to eat. They have so much to engage with. The competition for attention has never been higher.
Until a DC movie delivers a superhero story that is honest and confident, audiences will reject it. Batman V Superman, in all its heroic, Zack Snyder failures, is not the continuous, serialized storytelling people demand.
Thus far, it's just a brand exercise, and everyone who cares to look can notice.
Understanding The Avengers movies is a fun little club.
By now, people watching the movies in the Marvel Cinematic Universe understand the contract they've entered. Some movies are vital to the overall story. Some are only tangentially related.
It's a fun pop-culture history class that you don't always need to do the homework for. It's popular because the films permit different levels of engagement and enjoyment.
People who want to look for clues will find them. People who want to just follow Iron Man can do so. People who want to ignore Thor can do so, though I will recommend the first Thor movie because it's so gaudy.
A this encompassing continuity is fun because you get to know characters as individuals before getting to know them as larger groups. Anticipation and conversation build around what it'd be like if Star Lord met Ant-Man? Never mind the fact that those are two B- or C-tiered heroes, the audience was introduced to them in good faith through fun stories in their own right.
That's how you make something out of nothing. That's how you make continuity into a self-perpetuating story machine.
That's why this stuff is so popular.