What's the out-of-universe reason for the group's creation? There are at least three different explanations out there note that they don't contradict each other, so they might all be true for all we know:
The arguably most cited reason is that company owner/publisher Martin Goodman wanted a clone of Justice League of America, one of the Distinguished Competition's biggest hits. Well, another Expy, given that Fantastic Four had already come into being that way.
Another explanation is that Marvel's fans loved Crossovers, so Stan Lee was inspired to do a permanent one.
Finally, it has been said that Daredevil was meant to launch in The Avengers's place, but got delayed due to Bill Everett's personal issues. Because print time was very valuable back in the day and couldn't easily be canceled, a replacement was quickly created note that the first issue is exclusively about characters who already existed.
Prior to 2015, when the Fantastic Four comic was cancelled and he joined the Uncanny Avengers, the Human Torch was the only standard Fantastic Four member to not join the Avengers, and was in fact the only Stan Lee created character to have never done so. There have been many stand-in members of the Fantastic Four over the years that have also been Avengers: She-Hulk, The Incredible Hulk, Spider-Man, Wolverine, Luke Cage, Crystal of The Inhumans, Namor, and Ant-Man II.
The original, World War II-era Human Torch was also a member of the Avengers even though he isn't as famous.
In the Fantastic Four The End mini-series, Johnny is the leader of the Avengers in the future.
Approval of God: It was Roy Thomas who first had the idea of the Vision being the Human Torch in a redesigned body, but never got the chance to work on that story. It was Steve Englehart who finally wrote it down, in The Celestial Madonna Saga, with Thomas' blessings.
Old Shame: "The Avengers #200" is the single issue of the original comic line that ran from 1963 and into the 1980s that Marvel most wants to forget. It was so unpopular even when it came out in 1980 that Marvel immediately scrambled to address it in "The Avengers Annual #10" a year later.
Screwed by the Network: The first issues (of the Lee-Kirby era), translated in Spain. For some reason, Avengers #9 was not published, they moved from #8 to #10. Initially, nobody noticed anything wrong (at that point, Status Quo Is God), but in later issues fans began to notice recurring flashbacks about the death of a "Wonder Man", which was not in any of the issues they had.
Technology Marches On: In the first Avengers story, Rick Jones tries to send a radio message to the Fantastic Four about Hulk, but Loki prevents it with his magic, and redirect it to Thor's radio instead. And, by accident, it is also redirected to the radios of Iron Man, Ant Man and the Wasp. The crossover Avengers/Ultraforce, made in the 90s, included a new universe with a revised origin for the Avengers. The radio message was replaced with an e-mail (and yes, it also reached other people as well, somehow).
Why Fandom Can't Have Nice Things: When Jim Shooter wrote Avengers #213, where Pym attacked Jan (something that he Never Live It Down), he received loads of mail complainings. He turned to Stan Lee for advise. Lee told him that he got a similar backlash about Spider-Man's love relations back in the day, and asked about the sales. They were growing. Lee told him that means that he should ignore the mails; if sales were growing it can only be because people like the stories, despite the casual complainers.
Writer Conflicts With Canon: In Avengers #213 Henry Pym gave a right-cross to the Wasp. The scene became one of the biggest examples of Never Live It Down in comics, and forever influenced reader perception of Henry Pym. Writer Jim Shooter claimed years later that it was all a misunderstanding. He said "there is a scene in which Hank is supposed to have accidentally struck Jan while throwing his hands up in despair and frustrationmaking a sort of get away from me gesture while not looking at her. Bob Hall, who had been taught by John Buscema to always go for the most extreme action, turned that into a right cross! There was no time to have it redrawn, which, to this day has caused the tragic story of Hank Pym to be known as the wife-beater story". His explanation, however, does not make sense with other scenes. Before it, there is build-up to their relation being in crisis. After it, Jan shows up in the Avengers mansion, revealing a badly bruised face, to the horror of Thor ("Odds blood! her face! What's the meaning of this? Did... he strike thee, woman?"). In the last scene, the Wasp refuses to cry over what happened. In the following issues, the Wasp ends their relation and starts divorce proceedings. In issue 224 (still written by Shooter), Scott Lang sneaks into prison to hear Hank's version of the story, and Pym says "The tabloids got most of it right, Scott! I was an idiot! I let my personal problems overwhelm me! I got in trouble and took it out on... on Jan!". The arc makes perfect sense with the scene being what we saw, but makes little or no sense if the scene was what Shooter claimed it to be. Add to that that the artist Bob Hall pointed that Shooter never voiced him any complain about the scene.
Writer Revolt: After Mark Millar big success with Ultimate X-Men, Marvel proposed that he write a spin-off comic, Ultimate Wolverine, but Millar wanted to use the Ultimate The Avengers instead. Kurt Busiek, the writer of the Avengers at the time, did not want that to happen, as he feared that the regular Avengers would be left in the shadow of this new comic book. As the Ultimate Marvel universe was turning into a Cash Cow Franchise, so badly needed by Marvel to get rid of the risk of bankruptcy, they allowed Millar to work with the Avengers. The new team got a different name, as Busiek requested, and was named "The Ultimates". This was not enough for Busiek, who resigned from writing the Avengers as a result.