The Soul Stone might just be useful to predict the movements of other characters and then use them to perform an action that follows. According to Strange, when he looked to the 14 000 605 futures, only one the Avengers would win, meaning that, no matter what, Thanos was going to snap everyone (or at least half of it, but you get the idea). When he predicts it, I assume he already has the soul stone, or was in his way to get the soul stone. So whatever happens later, Thanos is going to predict the actions and/or control them, to help him give an advantage. So Thanos was going to win against Iron Man, Spider Man, the Guardians and Doc.Strange, no matter what. Maybe the Soul Stone had something to do with it. So Strange played accordingly to this fact, and lets Thanos win, gives him the Time Stone, and just hopes for, later, Ant-Man get out of the quantum realm, to which he would certainly tell the Avengers to time travel and get the infinity stones. Thanos wouldn't have predicted that. Doctor Strange beats Thanos in his own game, knowing that, no matter what, they were going to loose and the snap was going to happen.