By fronting his own baggage, Comey accomplished two objectives. First, he got to control the narrative about his decision to make his memos public and present it straight to the “jury”: the Senate. Remember that even if Special Counsel Robert Mueller finds enough evidence to bring obstruction charges against the president, most legal and constitutional experts agree that a sitting president can’t be indicted. He would first have to be impeached and removed from office under the process outlined in the Constitution, which has two stages: a trial in the House of Representatives, and a conviction by the Senate. The Senate would be the body ultimately weighing Comey’s credibility as a fact witness against the president, and they presumably now know the worst there is to know about his character.
Second, and more importantly, Comey made his story public so early that it will be a nonissue if he ever becomes a fact witness in a trial. Although public support for impeachment and removal of the president is growing (about 50-50, according to Vegas betting odds), any such action, if it happens at all, is months and more likely years away. Although Comey’s testimony is currently dominating the news, every possible legal and moral angle will be parsed ad nauseum over the next few weeks and eventually replaced with some other “scandal.” By the time this issue resurfaces again at some date far in the future, it will be snooze-worthy and unlikely to have any meaningful impact on the rest of Comey’s testimony.