There are 3 theories I know of so far as to it being used in the movie.
One: In the battle with Dr. Strange on Titan, he used it to locate the real Dr. Strange after Strange duplicated himself, because the stone glowed before all the duplicates were destroyed.
Two: In the end when Thanos snapped his fingers, he appeared in what looks like the soul world where he saw Gamora. Gamora was the first one to be sacrificed unto the stone so it’s ideal that she would be there and the atmosphere was orange as well.
Third: (this one I have speculation about) In the end when everyone was turning to ash, some are saying it was Thanos using the soul stone to kill everyone. I just thought it was the fulfillment of his wish, that half of life in the universe disappears.
Please share your thoughts on how you think it was used in the movie, if it was.
And do you agree with any of the theories or have any of your own?