The reason that this is important is that this guy who stops by for a drink is going to entrap Farquhar. That is pretty much his whole purpose -- to go around and entrap people who might be inclined to help the Confederacy against the Union.
When this guy stops by for a drink, he also lets Farquhar know about the bridge and how important it is and how there might be a good way to burn it down. When he does this, he puts the idea in Farquhar's mind and this leads Farquhar to go and try to destroy the bridge.
So the fact that this guy is truly a Union soldier leads to Farquhar's death.