I may have mentioned Fox News saying it was a simple case of Comey's word against Trump's.
But it's not that simple.
As an analogy, I used to write traffic tickets. If the driver went to court, I would have to testify under oath. After being sworn in, the judge would say, "Officer, state the facts". I would then, using my recollection refreshed by my notes, testify as to how I came to write the ticket.
Like it or not, if all the defendant did was argue the facts, he or she would usually be convicted. It's because officers are held to be experts who are sworn to uphold the law in a fair and impartial manner. Defendants can win, but will have to provide some evidence contradicting the officer's testimony - maybe a photo of a stop sign blocked by a branch, or something like that. And the state must prove its case beyond a reasonable doubt, so all a defendant has to do is provide something to bring the officer's testimony into doubt - not 100% prove him wrong.
Relevance? Comey will be assumed to be telling the truth. He's not just a traffic cop, he's a head cop and if it's his word against Trump's, his word will likely prevail - especially if he had contemporaneous notes and memos he shared with others. It may then be up to a Grand Jury or a jury of Trump's peers to assess the credibility of the Comey's notes, memos and sworn testimony. In a normal proceeding, they would carry a huge amount of weight.