He said why in his statement afterwards. He knows the amount of privilege he has as a rich American public persona, and wanted to use said privilege to see what these interrogations were actually like. He had a line he wouldn’t let them cross (searching his phone), at which point he would have shut up and gotten his lawyer, but the interrogation ended rather abruptly once he asked if he was being detained and they let him go. He shared some interesting anecdotes from his time in the detention area, and it’s good that this is being reported on instead of being done in secret.