Yamcha might be the butt of most jokes, and rightly so, but he once saved Goku from a bunch of regular thugs during the original Dragon Ball series.
Although the rise of insanely powerful aliens in Dragon Ball Z nerfed his character for the rest of the series, Yamcha is actually a capable fighter by human standards. He was therefore able to serve a much more critical role before DBZ when Dragon Ball's villains weren't so ridiculously powerful, rendering Yamcha completely useless. Yamcha proved himself capable on more than one occasion throughout these early years, but one of his most impressive exploits involved defeating a villain who Goku couldn't defeat on his own.
In Dragon Ball 17 by Akira Toriyama, Goku gets in a fight against one of Dragon Ball's most ridiculous characters, an anthropomorphic rabbit named Carrot Master who can transform people he touches into carrots. When Carrot Master performs this odd yet unsettling power on Bulma and threatens to eat her, Goku has no choice but to let the villain's thugs beat him up. However, Yamcha comes to Goku's rescue by defeating the thugs who had incapacitated Goku, and since he had his shape-shifting friend Puar successfully snatch Bulma from Master Carrot, Goku was now free to defeat the villainous rabbit.
This Victory Proves Yamcha is an Underrated Z-Fighter
Although Goku could have won on his own in theory, what made Master Carrot a worthy opponent wasn't his strength or fighting prowess but his ability to transform anyone he touched into a carrot. So Master Carrot using this skill to his advantage against Goku is akin to a massively strong enemy like Frieza besting Goku in physical combat. Goku was clearly unmatched in this capacity. Master Carrot's temporary victory also can't be attributed to the use of a dirty trick when said trick is his only method of fighting (unlike Captain Ginyu's Change Body ability).
The only problem is that Toriyama could have made Yamcha into more of a hero, as the mangaka creates the impression that Goku didn't need Yamcha to take down the thugs beating him up. The fact that Yamcha tells Goku that they just saved Bulma after doing so means that Goku was still only letting himself get beaten up because he was still under the impression that Carrot Master could eat her. Toriyama could have made it so that, in allowing the thugs to beat him up, Goku could have actually gotten hurt enough to warrant the need for Yamcha to save him. It's also unfortunate that Yamcha didn't possess any long-range weapons at the time so that he could safely defeat Master Carrot. The fact that Yamcha also tells Goku to use his staff means that he knew that Goku could defeat him at a distance without running the risk of getting touched and subsequently transformed into a carrot.
Regardless, Yamcha is clearly the brains behind the whole operation that resulted in Master Carrot's defeat. He uses his knowledge of Puar's abilities to rescue Bulma so Goku would be able to safely battle Carrot Master without having to worry about Bulma getting eaten. Then Yamcha employs his physical capabilities to save Goku from the precarious situation that Carrot Master had forced him into. If Yamcha had not been there, then Carrot Master's thugs could have easily beaten Goku unconscious and then transformed Goku himself into a carrot, which would have made the rest of Dragon Ball very different.