Stan Lee created an incredible universe of heroes, but what made the business a success was his ability to convince buyers they were missing out on some element of their favorite characters story line if they didn’t buy every comic published by Marvel:
To ensure that readers would keep coming back for more, Lee mastered the concept of “continuity”—the idea that different comic book titles were interconnected, all taking place within a single “Marvel Universe.” He was able to do this because, in the early 1960s, he was editing and scripting the stories of practically every Marvel character, including the Fantastic Four, Thor and Spider-Man. … But it was a business decision as well: Those interconnections—signaled by asterisked footnotes within the comics—alerted fans that they would be missing out if they didn’t buy all of Marvel’s titles. Today, the same model applies to Marvel movies, where an “Easter egg” scene in the credits might be used to tease an upcoming film or new character.
It perhaps seems obvious today with the Marvel Cinematic Universe (now led by Walt Disney Studios which acquired Marvel Entertainment for $4 billion in 2009) weaving every character together in the Avengers franchise, but it was a novel approach at the time. Follow the link for more details describing how Stan Lee built his empire.