Doctor Steven Strange started off as a neurosurgeon, but after an accident injured his hands, he sought out the Ancient One. Under her tutelage, he learned about the mystic arts. By the time he is seen in Avengers: Infinity War, he seems to have mastered the use of multiple magic spells.
He even has a better understanding of the Time Stone that he was in charge of protecting. If he had been introduced earlier, and had obtained his powers earlier, he would have been crucial in helping the Avengers save many civilian lives. He could have also helped the Avengers avoid certain events that had tragic consequences.
10 He Could Have Helped Scarlet Witch Master Her Powers
After Wanda Maximoff's parents were killed in a bombing, she and her brother Pietro decided to join HYDRA, willingly going through experiments utilizing the mind stone. This amplified her Chaos Magic, which she would be seen using often during and after the battle against Ultron.
Doctor Strange would have been a good mentor to her if he had approached her, as he is able to use magic and Wanda had no idea what her full potential was. This might have helped her avoid many of the events of WandaVision.
9 He Could Have Fixed Tony's Palladium Poisoning Much Earlier
As a neurosurgeon, Doctor Strange would have had the talent to save Tony's life sooner. He learned the things that he knows quickly, thanks to the fact that he has a photographic memory. His magic allows him to move his hands and allows him to duplicate himself and use transmutation.
His skills would have kept the shrapnel from getting into Tony's heart during the operation and would have prevented the palladium issue.
8 He Could Have Stopped Hulk's Rampage In The Helicarrier
After Bruce Banner was tricked into transforming into the Hulk on the helicarrier, he began to chase after Black Widow, blinded by rage. She and Thor were unable to stop his rampage, but Hulk jumped onto a nearby jet that was shooting at him, which got him safely away from the helicarrier.
Doctor Strange would have been able to transport Bruce without ever putting him on a helicarrier that he could easily destroy.
7 He Could Have Stopped Hulk's Rampage In Johannesburg
The Hulk rampaged a second time, after Wanda tampered with his mind. He started attacking the city of Johannesburg, and Tony Stark had to rush to stop him with his specialized Hulkbuster suit.
Despite his attempts to calm him down and to reduce the damage to the city, they both ended up destroying a lot in their battle until Tony was finally able to knock him out. Doctor Strange would have had a hard time matching Hulk's strength, but he could have at least moved the battle to a better location.
6 He Could Have Fought Aliens With The Other Avengers
When Loki invaded New York City with a portion of Thanos' Chitauri army, the Avengers fought them in one of the biggest and most difficult battles of their lives. Despite each of them being a capable fighter, there were only a few of them against an entire army.
Any extra Avengers would have made the battle much easier. Doctor Strange, who put up a good fight against Thanos himself, would have made a huge impact in the Battle Of New York.
5 He Could Have Helped Fight Ultron
Ultron was a huge threat, as he managed to lift the entire city of Novi Grad, planning to use it like the asteroid that killed the dinosaurs. He had an army of countless sentries that shared his consciousness and attacked innocent bystanders in the floating city.
An extra Avenger could have reduced the casualties of nearly 200 people, and Doctor Strange would have had an easier time protecting the device to trigger the destruction. He didn't have the same attachments that Wanda did, given that she abandoned the device to avenge her brother.
4 He Could Have Prevented Civil War
The Civil War amongst the Avengers, as well as the Sokovian Accords that started the conflict, could have been avoided if Wanda had not been the one who tried to stop Crossbones. Crossbones tried to use his vest to blow himself up along with Steve Rogers, but Wanda lifted him into the air and accidentally moved the impact into a building filled with civilians.
For Doctor Strange, transporting Crossbones away would have been child's play, as he could have just opened a portal underneath him and sent him to an unoccupied area.
3 He Could Have Searched For The Stones Sooner
Thanos spent a long time scouring the universe for the Infinity Stones, and thus, had the advantage. If Doctor Strange tried to search for the stones around the same time, he might have been successful in finding them. He was able to open portals to transport people from Titan to Earth, meaning that he could have visited many planets faster and easier than Thanos did.
He also has ways of hiding the stone, as he was able to hide the Time Stone from Thanos until he made it reappear in order to hand it over.
2 He Could Have Helped With Any Evacuation Efforts
When it comes to getting people from point A to point B in an instant, Doctor Strange is one of the best. During the attack on New York or the battle against Ultron, Doctor Strange could have evacuated a large number of people, drastically reducing casualties.
If he and other sling ring users could transport an entire army, they could transport just as many civilians to a place far away from the main battle.
1 He Could Have Extracted The Aether From Jane Foster
When examining an anomaly, Jane Foster was pulled into a portal that brought her to the Aether. The Aether, which is also the Reality Stone, entered Jane's body. When Thor discovered this, he took her to Asgard, thinking that no one on Earth could help her with her ailment.
Ironically, no one on Asgard could either, and they had to rely on the Dark Elves to remove it. Doctor Strange could have made a difference here, as he has the Time Stone. Given that Earth has the Avengers, and that Odin was crushed after Frigga's death, Earth would have defended itself better from the Dark Elf invasion.