It's hard to imagine Dragon Ball Z or Dragon Ball Super without Androids 17 and 18, who have become iconic central characters in the long-running franchise. Android 18 is married to Goku's oldest friend, Krillin, while Android 17 saved the entire Multiverse from annihilation. The two androids have gone from villains to heroes and parents, so it may come as a surprise that their introduction into the Cell Saga was not initially part of the plan.
From the moment Androids 19 and 20 appeared in Dragon Ball Z, it was clear there was no way they could be the main villains of the next saga. The reveal that the androids who had pushed mankind to the brink of extinction in Trunks' future were an old man and a rounded mime(?) was underwhelming, to say the least. Fortunately, Kazuhiko Torishima, the editor of the DBZ manga at the time, agreed and pushed Akira Toriyama to create new androids who didn't look so "outdated and ridiculous."
Androids 17's and 18's designs were much more suited to the series' new aesthetic at the start of the Cell Saga. The deadly pair were still intimidating and a match for the Z Warriors, whereas 19 and 20 were silly-looking and surprisingly weak. Revealed to be the true androids that decimated Trunks' future, the two villains were first an insurmountable obstacle for our heroes before becoming the keys to Cell's evolution. Their youth went a long way in instilling a sense of sympathy as the two fought to resist being absorbed by Cell.
Android 18 was a great introduction for Krillin, given that DBZ to this point had treated one of its oldest and most iconic characters as a human punching bag. This character development for Krillin was crucial, as he was becoming increasingly irrelevant once it was clear he would not be keeping up with Goku's power level. Marrying Krillin and giving birth to Marron also humanized Android 18 and make her sympathetic enough to believe that no one would hold a grudge against her once she joined the main cast after the Cell Games.
Android 17 was MIA for a long time after the conclusion of the Cell Games, but it's safe to say that Dragon Ball Super would not be the same without him. Unlike his sister, 17 had yet to receive any redemption for his actions during the Cell Saga, but saving the entire Multiverse more than makes up for any harm he did after his introduction in DBZ. His infinite stamina and quick thinking were vital to Universe Seven's victory, and it's difficult to believe that anyone other than him could have filled the role he played in the Tournament of Power.
Thank Zeno for editors, because who knows what Dragon Ball Z and Dragon Ball Super would have become without Androids 17 and 18. Goku seeking out a reformed Dr. Gero for the Tournament of Power would have been awkward, and it seems unlikely that any of the Z Warriors would have married Android 19. In a series that's become hyper-focused on Saiyans, it's refreshing to have warriors like 17 and 18 who fight differently, but no less competently. The fact that the two androids are also both devoted parents and full-fledged heroes is just the icing on the Senzu Bean cake.
