Big Bird had long talked about his best friend Snuffy, but he was always believed to be just a figment of his imagination, an imaginary friend that the adults on Sesame Street thought he used as a scapegoat whenever Big Bird made a mistake or did something wrong. However, Snuffy finally was revealed to be real in 1985.
I'd always seen this decision as, like, making a statement about the imaginary friends of children being real to them and that those relationships should be valued by the adults in their lives. A valid point, to be sure -- and Snuffy was an adorable addition to the cast of muppets -- but I've come to read that the real reason the show decided to actualize Snuffy due to a string of high profile and sometimes graphic stories of pedophilia. The writers on the show worried that young viewers would learn that adults wouldn't believe them, even when they were telling the truth, and in cases of sexual abuse, would be better off staying silent. When Snuffy was revealed to be real, the adults on the show made a point to apologize to Big Bird and assured him that they would always believe him from now on.
This level of care and consciousness about how and what their young viewers were capable of learning is one of the reasons why "Sesame Street" has been such an important show for kids, beyond just teaching them their ABCs.