The Avengers have done a lot for Marvel in their multi-decade existence. The team has managed to change the course of the Marvel Universe many times over the years, making it a place where readers wanted to spend a lot of time.
Captain America is among Marvel's most accomplished heroes, in large part because of his association with the Avengers. Cap was Golden Age Marvel's greatest hero, but he was completely absent from the publisher's Silver Age superhero relaunch until Avengers (Vol. 1) #4. This book by Lee and Kirby brought the legend back for a whole new generation of fans.
Cap's return changed the Marvel Universe forever. He became the superhero community's de facto leader, the shining example who all others looked up to. It's impossible to picture the modern Marvel Universe without him.