It is used for income targeting in ads. Sorry about the lousy/spammy reference, but it's the best explanation I could find.

From [1]:

"Partner categories are, importantly, only available in specific areas. Acxiom provides data from Australia, France, Germany, the US, the UK, and Japan. CCC Marketing provides data from Japan. Epsilon provides data from the US. Experian provides data from Australia, Brazil, the UK, and the US. Oracle provides data from the UK and the US, and Quantium provides data from Australia."

[1] https://boostlikes.com/blog/2017/08/target-facebook-users-in...

Ya, that's exactly what these data brokers do. Its anonymous(when in FB ecosystem) to the end advertisers but they sell your data to ad platforms or advertisers.