This is the first of one of Star Trek's most famous recurring themes: moralizing on the nature of war. It's a famous example, too, even if you don't realize it by the title alone. "A Taste of Armageddon" is the one where a planet runs its wars by computer (but expecting populations to accommodate them with real sacrificial victims).
Also, it's the first instance of Starfleet patronizing the natives, assuming it can fix something because it believes it's wrong. Something something *cough* Prime Directive *cough* maybe? Who knows? It happens in so many other episodes, apparently the writers never really draw a line between the distinctions of talking about war and general cultural interference.
But it's still extremely relevant, important to the series and franchise as a whole, although I will limit my viewing recommendation to only the series rather than inflate the episode's importance to something other than a strictly historical nature. It's important to acknowledge, but you don't need to go out of your way unless you really want to be a completist. That's to say, "Taste" is only as significance as the precedence it sets. It doesn't have a ton of memorable value outside of that.
Still, because the notion of a society being so coldly pragmatic about the nature of war (and not, incredibly, turning out to be related to Vulcans) is so striking a concept that for a change, the story alone really might be the hook, though it's not liable in itself liable to convince someone to like Star Trek in general (that's the reservation I hold for higher rankings, at least as a working theory; fans themselves have dismissed out of hand some of my selections for new classics, but I'm not making this survey strictly for existing fans, but rather as a window for new ones).
franchise * series * essential * character