Gran Torino may seem like just another side character who somehow scares the living daylights out of All Might and enjoys pretending to be a senile old man. However, he has arguably been the most aggravating thorn in All For One's side for a great many years.
Not only does Sorahiko Torino resemble Yoda in stature (and some may say, appearance, minus the ears), but the pair share a similar role in their respective storylines, too, as the inspirational mentors behind the world's greatest heroes. And this esteemed speedster has personally supported a third of One For All's Users, arguably making the greatest contribution to this Quirk's longevity in My Hero Academia history.
Third Generation: Izuku Midoriya
Gran Torino essentially saves Deku's life, by teaching him the vital skill of self-restraint. The young Hero tends to be quite trigger-happy and often unleashes more of One For All's power than his body can handle, which has truly detrimental results. All Might has also mollycoddled Izuku for the most part, so "Mr. Torino" swoops in to deliver some much-needed tough love, giving him the training experience of a lifetime.
Deku doesn't just learn how to control his power output during his short time with Gran Torino in Season 2, but he is fortunate enough to improve his strategizing skills too and learns to read the situations quickly and efficiently, treating the battlefield like a chess board. Fighting against such a fast foe requires one to hone their instincts, as there is little time to stop and think about the next move (whereas Deku previously had a tendency to overanalyze everything, wasting precious time as a result.) Izuku walks into Sorahiko's home as a hopeful rookie with unrealistic expectations, and walks out as a prospective top Pro-Hero, with a better head on his shoulders. Without this crucial step in his journey, Deku would arguably still be causing too much self-inflicted damage during conflict, and might even be crippled or dead by now (which would obviously make All For One's mission a whole lot easier!)
Second Generation: Toshinori Yagi
Sorahiko became All Might's de-facto mentor and father figure following Nana's death, taking full responsibility for the younger's hero training. Sure, he may have been rather harsh and strict with his student, but young Yagi arguably wouldn't have become the greatest Hero in history and an inspirational Symbol of Peace if Gran Torino hadn't guided him along the way. Assumedly being the only other person with an understanding of One For All, and the associated dangers, Sorahiko was the only one capable of effectively preparing All Might, pushing him to the limits of his capabilities, and beyond. Yagi was still quite young when Nana was taken from him; if left to his own devices or trained by an ignorant sensei, All Might's claim to the #1 Pro-Hero title would have been just a dream.
Even after graduating from UA and becoming a Pro-Hero himself, Yagi still relies on Gran Torino for council and assistance (although he is clearly still traumatized from their training days!) Thankfully, their relationship seems to have been revitalized since Bakugo's kidnapping in the Hideout Raid arc. This has inspired Gran Torino to rejoin crime fighting and assist All Might, the Police Force, and the likes with some vital behind-the-scenes work, all in the name of defeating All For One, of course!
First Generation: Nana Shimura
Specific details regarding Nana's past are still a little hazy; however, fans have been provided with enough flashbacks to piece together a pretty decent picture. Just like the previous Users (except for Hikage Shinomori, the recluse), Nana made it her life's mission to eradicate evil from the world and defeat All For One. She assumedly met Gran Torino through her line of work (although the fan theories run rampant!) and the pair formed a deep bond; one so strong that Nana trusted Gran Torino with her most confidential secrets. Not only is he privy to the existence of One For All, but he also used to get regular updates about the Quirk's development and subsequently has quite a thorough understanding of its mechanics (lessons which he hands down to the future wielders.)
After passing the vibe check, the handsome young(er) man quickly got on board with the mission and regularly teamed up with Nana, offering her guidance and support in the battle against All For One. And not to say that she didn't have any other allies, but Sorahiko (and later, All Might) is pretty much the only friend to feature in her flashbacks. Therefore, it is safe to say that Nana's strength, determination, and savvy tactics were boosted significantly by her close connection to the speedster, who she trusted with her life.
Furthermore, the ever-smiling Hero might have fallen into an inescapable pit of despair after her husband's death and the whole adoption debacle, rendering her useless on the battlefield. Furthermore, she would plausibly be lacking in any ambition to pass her Quirk on to the next generation or become a teacher at UA. And, most importantly, One For All would have died with Nana and All Might if Gran Torino hadn't come to the rescue and extended a lifeline to her successor (and his precious, powerful cargo.)
The Big Man Himself
All For One is evidently cautious around Gran Torino. Why else would he send all his minions through a warp gate (in Season 3, episode 10) the moment Sorahiko arrives on the battlefield when the League of Villains clearly outnumber the Pro-Heroes? Yes, saving Shigaraki is the priority, but when has All For One even given up the chance to cause some extra bloodshed? Gran Torino finds a gap amidst the chaos to give All Might a much-needed pep-talk during his crucial fight against All For One, helping to calm him down before things end disastrously. And with that, the experienced Hero earns himself a front-row seat to The Symbol of Evil's defeat after contributing to his capture. All Might's trickery is all thanks to Sorahiko's wise words, which catch All For One completely off guard.
One could make the counterargument that if Torino had convinced Nana to keep her family close and raised Kotaro herself, then maybe Shigaraki wouldn't have inherited his father's hatred for Heroes. Perhaps they would have sought vengeance for their fallen matriarch, and turned their anger towards All For One instead, drastically changing the game for all involved. However, this argument has a few pitfalls.
Nana's death could have still inspired resentment in the Shimura household, because why didn't Torino or the other Heroes save Nana, instead of leaving her to die? Furthermore, Shigaraki would have still been born with the same Quirk, and potentially killed his family (and/or other innocents) regardless of his mental state, leaving him vulnerable to All For One's manipulations. Perhaps Shigaraki was predestined for destruction all along, and the Heroes could do nothing to stop his descent into darkness. However, Gran Torino hasn't made the Villains' lives any easier, he just doesn't demand much praise for his crucial role.