In the beginning of Thor: Ragnarok, Thor said:
Well, sometimes you have to get captured just to get a straight answer out of something.
What exactly did he want to find out?
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Thor wanted to find out more about the Ragnarok prophecy. And, he succeeded in doing so.
Surtur told him that when his crown would be put inside Eternal Flame, he would grow bigger than a mountain to destroy Asgard. Thor used this information to knock Surtur's crown out of his head so that it could never be put into the Eternal Flame.