What's the function of 所 in 他被这个问题所困扰。?

I have this in my flashcard:

He was perplexed by this problem. Tā bèi zhè gè wèntí suǒ kùnrǎo. 他被这个问题所困扰。

I don't understand what it does there.

Thanks in advance.

submitted by /u/sanyides
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