While Goku and the Z Fighters' time on Namek is best known for the introduction of the Super Saiyan transformation and Goku's epically long duel against the interstellar tyrant Frieza, plenty of details in the lead-up helped make the Namek Saga one of the most vitally important storylines in the entire Dragon Ball franchise.
Moving several characters out of Goku's shadow and onto paths that would define them for the rest of their lives, the Namek Saga escalated Dragon Ball Z's stakes and scope while giving the story some memorable characters and moments that continue to shape the mythos decades later.
One of the Namek Saga's most striking features is that the familiar cast of characters is shaved down considerably after the Saiyan Saga, while Goku himself was largely sidelined as he recovered from his injuries after fighting Vegeta for the first time. This led Gohan, Krillin and Bulma to lead much of the narrative, with Vegeta joining proceedings once the Z Fighters arrived on Namek. With Gohan having overcome the anxiety he suffered during the Saiyan Saga, the young fighter really came into his own with this storyline, especially after having his latent potential awakened by Guru.
A major surprise during the Namek Saga came from Vegeta -- originally established as an antagonist -- who was now forced to run and hide from Frieza and his most elite warriors. With the Namek Saga revealing the true story of how the Saiyans lost their home planet and were brought close to extinction, the revelations had the Saiyan Prince performing hit-and-run attacks on the Frieza Force. This culminated in Vegeta working with the Z Fighters for the first time, desperately teaming up after Frieza called in the Ginyu Force to deal with the opposition to his plot. Though still a ruthless killer, this alliance marked the beginning of Vegeta's gradual redemption ahead of his return to Earth.
Then there's the matter of Frieza himself. No other villain introduced over the course of the Dragon Ball franchise has quite the staying power as Frieza, showing up in smaller appearances either cybernetically resurrected or languishing in Hell before his full return at the start of Dragon Ball Super. While the full extent of Frieza's power and brutality wouldn't be unveiled until Goku arrived on Namek to face him, his introduction set the new standard for major antagonists in the franchise that arguably hasn't been topped since. Balancing Frieza's tyrannical presence was that of the young Namekian Dende, who would play an important role in the franchise moving forward as Kami's successor, complete with his own set of Dragon Balls.
With Goku sidelined and the Z Fighters outmatched, the Namek Saga was an inventively paced and staged storyline for Dragon Ball Z, allowing the more secondary characters the chance to step up in a big way. And while new, unlikely alliances were forged and important enemies and allies were introduced, the narrative finally left Earth to expand across the cosmos. With the fate of the universe now hanging in the balance, DBZ's stakes were the highest they had ever been and have rarely subsided since, setting the bar for the story moving forward. The Namek Saga did far more than set up the epic brawl between Goku and Frieza -- it informed all of DBZ and its sci-fi sensibilities moving forward.