No, I don't mean that their role in the army is medical; it's not.
I mean that, if people train for fighting *or* train for healing, heralds are the latter.
Remember how Finrod and Edhellos had a conversation in Episode 4 about how she had 'stepped into' her role in her husband's army, and how Finrod seemed a bit surprised by that? That's what we want the herald to be - someone who is a bit of an outsider to the fighting, but still important in decision-making. They hold a high enough rank that they aren't expected to do much actual fighting, though they can take part in fights/battles.
In the current US Marine Corps, lieutenants are officers fresh out of school, so they don't have much practical experience yet, and they're commanding enlisted men who do have more experience and training when it comes to fighting. What they bring to the table is meant to be a bit more of the tactical side of things and formal training rather than the practical side of things. That's what their sergeant is for; someone older and more experienced than the lieutenant, but lower ranking and not an officer. (If you want to be cynical about this, it's the sergeant's job to make sure the lieutenant doesn't get anyone killed due to his own lack of experience.)
We're going for a rather different dynamic here, but we still want that duality of each person bringing a different skill set to the table, and for the elven culture to not 100% value skill at armed combat above all other skills.
Elrond is a healer, and Elrond is the herald of Gil-galad. We know that Elrond commanded the army that confronted Sauron in Eregion; Gil-galad wasn't even there. So, it's not like Elrond doesn't fight. But he did recognize it was a losing battle and went ahead and founded Rivendell instead. So, you know, that's the kind of outside-the-box thinking that you'd expect from a herald
(Also, as an aside, we're going to use the word 'lieutenant' at some point in this story, so there's nothing wrong with introducing it here.)