Why did US create a secret backchannel with Iranian guards?

When US began direct talks with Iran in May, Trump administration struggled to determine Iran’s true position. It then set up a secret backchannel with Iran’s IRGC to verify whether negotiators Mohammad Bagher Ghalibaf and Abbas Araghchi were actually speaking for Iranian military. Iraqi leader Nechirvan Barzani was chosen as intermediary because he was trusted by both US and IRGC.