Though it started out as more of an adventure story, the Dragon Ball franchise eventually became far more focused on pure action and intense battles. This created a dichotomy where the strongest characters received the most prominence while the others slowly began to fall by the wayside. Thus, former mainstays like Yamcha, Tien and Krillin became irrelevant as the series focused more on the Saiyans, namely Goku and Vegeta.
Due to this emphasis on martial skill, one prominent character in Dragon Ball Z never quite gets his due. Though Dende is never physically involved in a fight throughout the series, he's quite often crucial to the storyline and the universe's fate. Here's how Dende has continued to play an important role in the Z fighters' survival.
Dende Speaks The Namekian Language
In order to summon and make wishes from Planet Namek's dragon, Porunga, it's necessary to be able to speak the Namekian language. For this reason, the Z Fighters quickly attach themselves to the young Dende, who they would clearly need for such a task.
Likewise, Dende introduces Gohan and Krillin to the Namekian lord Guru, thus making him instrumental in the two fighters' true potential being revealed. His ability to speak Namekian and summon Porunga is vital not only in the Frieza Saga, but later throughout Dragon Ball Z -- especially in times where the Earth's Dragon Balls have been rendered inert.
Dende's most useful attribute is his ability to heal, which he gained while being shown his own true potential by Guru. This played a key role in the fight against Frieza, where he would stay on the sidelines to heal the wounded Z Fighters after their attempts to take the galactic tyrant down.
He even aids the violent Vegeta -- who had recently killed many innocent Nameks -- due to the team needing the Saiyan prince to help take down Frieza. This quickly draws Frieza's ire, who kills Dende after transforming into his final form. Eventually brought back to life via the Earth's Dragon Balls, Dende remained a staunch healer for the duration of Dragon Ball Z, such as when he restored the bruised and battered Gohan to normal after the Cell Games.
Dende's Heroism and Role as the New Kami
Despite not having a warrior's bone in his body, Dende is extremely valiant and heroic, standing in the face of sheer destruction several times. He never runs away during the fight with Frieza, remaining a steadfast sideline healer even at the cost of his own life. He also stands up defiantly against Vegeta when the Saiyan demands immortality. Likewise, he informs Hercule of what's going on and essentially keeps him safe during the fight with Super Buu, narrowly escaping many times throughout the saga with his own life.
It's for this reason, as well as his other aforementioned abilities, that Goku handpicks Dende to replace Kami as Earth's guardian. This makes him even more vital to the series -- subsequently tasked with recreating the Dragon Balls, the series' titular MacGuffins were now tied to his very life. Thus, though he never threw a punch the entire time, Dende became perhaps the most important character in Dragon Ball Z.