The Simpsons' Lenny Leonard and Carl Carlson are two of Springfield Nuclear Power Plant's workers. Introduced in the show's first season, the pair were portrayed as background characters at Homer's work, usually appearing together in minor roles. They often played into gags at the Power Plant, essentially serving as Homer's chief work friends and sounding board. However, as The Simpsons progressed, they increasingly appeared at Moe's Tavern, where they became more constant fixtures in Homer's day-to-day life, partly due to another character's decreased role.
As The Simpsons entered its second decade, Lenny and Carl received more and more characterization. Lenny became defined by his boring nature, tendency to suffer eye injuries and receiving an unusual amount of appreciation from the rest of the Simpson family. Meanwhile, Carl became more of a snarky figure at Moe's, known for his cunningness and sometimes sleaziness. Both have master's degrees in nuclear physics (as revealed in Season 8's "Homer's Enemy") to contrast the far less qualified Homer. There's also a romantic subtext to their relationship, especially on Lenny's part, who frequently talks about how great Carl is.
The pair later got an entire episode dedicated to Carl's origins as an adopted man from Iceland and their friendship, exploring the idea that Carl doesn't value it as much as Lenny. In modern episodes, the due can often be found walking around Springfield, serving as an easy set of eyes and ears to witness the weirdness occurring around the town.
As a result of their frequent appearances, Lenny and Carl became major fixtures in The Simpsons. The pair have proven to be a solid comedy duo. However, that might not be the biggest reason they got so much prominence, but rather, because of Barney Gumble more or less losing his role in the series.
Barney Gumble was Homer's "best friend" in The Simpsons' early season. He served as an even more over-the-top alcoholic whose life and choices could be considered goofier and stupider than Homer. But as the earlier episodes fleshed out his history, Barney increasingly became a tragic figure. Homer's influence was revealed to have played a big part in Barney's descent into decadence. While Homer always had enough restraint to have a family, Barney was portrayed as someone who couldn't overcome his addictions. He even constructed a short film about his alcoholism in Season 6's "A Star Is Burns." However, he wastes his new chance to embrace the grand prize of a lifetime amount of Duff Beer.
As The Simpsons progressed, Barney became a far less prominent character. In modern episodes, it's rare to even see him on "friend outings" that include Lenny, Carl and Moe. Despite this, the show still wanted to let Homer have some close friends to bounce off of, allowing Lenny and Carl to be utilized across the rest of the series. It's a surprisingly sad reason. Their ascent into stardom parallels the descent in Barney's relevancy, suggesting that Homer spends more time with his work friends than his longtime best friend. As such, episodes have been retconned to allow Lenny and Carl to grow up alongside Homer, while his best friend from high school ends up on a much sadder path.