I think that doctor strange gave away his time stone to save Iron man because when doctor strange saw all the consequences from the fight with Thanos, he viewed one consequence where Iron man flicked his fingers to eliminate Thanos’ army. Also, that’s the reason why doctor strange realized that the vanished avengers needed to go and help the left avengers after 5 years.(spider man told tony stark at the endgame)In addition, a lot of people’s thoughts conflicts in this question, “What’s the meaning of doctor strange’s finger pointing upward?”and my opinion is that doctor strange wanted to say to Iron man “This is it! Get the stones and flick your finger!”. And Iron man flicked his finger to eliminate Thanos’ army and the avengers won!