The Marvel Cinematic Universe was created, designed, built and marketed around the superheroes that headlined each solo movie, but the key to maintaining a successful shared mythology is surrounding them with memorable supporting characters that audiences can believe and invest in.
It would be almost impossible to imagine Batman without Alfred, Harry Potter without Ron or Hermione, or Woody and Buzz without the rest of the Toy Story gang. The name on the poster might be what sells a movie to audiences, but it's the network of background players around them that keeps people coming back.
The Avengers don't exist in their own little bubble and save the world without any assistance, despite how self-absorbed many of them seem to be at times, and the MCU's extensive back catalogue is full of side characters that have made their own indelible mark on the series at various points.
Just because they don't get to be the one suiting up to fight the bad guys, it doesn't mean they don't have an impact on events, both present and future.
I don't do social media, so like or follow me in person but please maintain a safe distance or the authorities will be notified. Don't snap me though, I'll probably break. I was once labelled a misogynist on this very site in a twenty paragraph-long rant for daring to speak ill of the Twilight franchise. I stand by what I said, it's crap.