Comey basically had two options and believe it or not he chose the option that was less upsetting for Democrats. The NYPD presented new evidence relating to the investigation on Clinton's server to the FBI in the form of a laptop that they found in an underage sexting scandal involving a US congressman.

He could have reviewed the presented evidence without informing Congress (not a statement released by the way) and it would have leaked that Comey announced publicly that he ended the Clinton investigation and then almost immediately re-opened it secretly. Or he could have been honest with the country, while being very cautious with words and openly describing his hesitance to do so, because that his fucking responsibility as the Director of the FBI. So that's what he did.

The credibility of the FBI comes from its perceived objective reign of law enforcement on all levels. We would be completely fucked if there was no one to investigate politicians and rich people, and the FBI doing incomplete investigative work on a major scandal for bi-partisan political appeasement is absolute political horror fiction.

And anyway, The FBI are the guys who find kidnapped kids, bust corrupt politicians, help small town cops with murder cases they're not qualified for, all that shit. The CIA are the guys that destabilize millions of peoples lives regularly for socioeconomic gains, dose people with LSD until they become domestic terrorists, radicalize people who go on to assassinate US presidents, cooperate with the mafia and cults for whom the CIA has ended FBI investigations into for child trafficking. These groups go back way further than the last 6 months, people shouldn't be so shallow to think the reputation of these groups hinges in any way on whether or not bias was shown to a political preference of yours.