The CIA, together with the military, serve "America's interests abroad" (though not always abroad). That catchphrase "America's interests" is shorthand for the interests of the wealthy, privileged and powerful; both individuals and corporations historically. Epstein cultivated relationships with those people to get rich and keep getting richer. Of course, he got rich the old-fashioned way: he stole it.(1) Epstein sees to have used investment schemes that evaded taxation to lure folks into his clutches. If you understand rich people, you know that the last thing they think they need to do is pay taxes, so he had lots of takers. That gave him lots of connections that the CIA could cultivate. I would also suspect that the digger one digs, what the CIA did FOR Epstein in exchange will likely become clearer, if it's not buried as "national security secrets." I would expect things to get even more sordid.
Pedo Island and such are just the next level of kompromat that Epstein could use to his ends, whatever they may have been. I expect that was also occasionally useful to the CIA and others. Mossad and the GRU have been hinted at, and Epstein seems the type to have taken money and "benefits" from anyone. Digging into these connections also may reveal why he was murdered in jail.
BTW, if you don't how sordid the history of the CIA is, and precisely HOW they serve the interests of America's elite, read The Devil's Chessboard by David Talbot. (2)
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(1) https://www.dw.com/en/where-did-jeffrey-epstein-get-all-his-money/a-75944096
(2) https://books.google.com/books/about/The_Devil_s_Chessboard.html?id=QLT-rQEACAAJ&source=kp_book_description