Very much like the Voyager episode "Jetrel," we learn a disturbing history behind a species we've basically just come to know. Unlike the Talaxians being victims in that instance, Denobulans are the villains as featured in the revelations of "The Breach."
Unfortunately, Enterprise never got around to exploring Denobulans, the race of Dr. Phlox, too extensively, so this remains one of the signature expositions for them. As in "Jetrel," much of the episode is a reluctant dialogue between representatives of two species who were once at violent war with each other. In the one positive note of this exchange, it's never exaggerated to anything other than the doctor-patient relationship that it begins as.
The main benefit is learning a little more about Phlox's family. Earlier in the season we learned that he doesn't have the best relations with all his children, and here we learn why. It's a sobering spotlight for a character who often served either as comic relief or a voice of reason, perhaps more somber than is enjoyable, considering that when utilized Phlox was always among the best assets of the series. Like Mayweather before him, perhaps the wrong lessons were taken from the learning steps of Phlox's episodes from the early seasons. Instead of taking his place as the featured character he deserved to become, the good doctor became a thoroughly supporting character. It's not hard to view "The Breach" as a reason why.
On the plus side, the B-story does allow Denobulans to show a more positive and typically eccentric side, as some of the crew attempts to rescue some scientists from a tricky spot. It's an odd mix, but it counts as a thoroughly Denobulan affair, and this is the only episode that can say that.
franchise * series * essential * character
Memory Alpha summary.