Blackmail is one of the most effective tools to control people in power, especially politicians. Is this why Comey didn’t recommend charges for Hillary Clinton on the email investigation? If Hillary was aware of the FBI wiretapping, she could use that as blackmail against Comey.
It appears that Hillary Clinton was aware of the wiretap because she tweeted about it one week before the election.

We know that AG Loretta Lynch also met with Bill Clinton on the tarmac of Phoenix Sky Harbor International Airport in June of 2016 right around the same time as the first FISA request. Bill Clinton then passed the information about the wiretapping plan to Hillary Clinton in order to help his wife win the election.
If Hillary Clinton had knowledge of Comey’s involvement in the wiretapping, perhaps he let her off the hook to save himself from Clinton blackmail tactics.
http://www.thegatewaypundit.com/2017/03/blackmail-comey-let-hillary-off-hook-knew-fbi-wiretapping/