Reaction rolls were a revelation when I first realized what they are and how they work. People always think Dungeons and Dragons is only about fighting, and then you have actual social mechanics in older editions that somehow got disregarded over time.
You see, in older versions of Dungeons and Dragons reactions of monsters encountered were rolled (partially based on Charisma) and fights were not the necessary outcome. The reaction table looks like this (with variations according to the system):
2d6 Result
2 or less hostile, attacks
3-5 unfriendly, might attack
6-8 neutral, uncertain
9-11 indifferent, unhelpful
12 or more friendly, helpful
That means a monster outright attacking is a rather rare occasion. It’s a possibility though. That is, unless your charisma modifier is high enough to allow for more beneficial results.
Any other result than the lowest likely gives characters some way to deal with the monsters that does not involve violence.
The exception are of course enemies that are attacking outright, like some undead, or enemies that are targeting the characters specifically.
In addition there also are morale rolls. Morale rolls are a way to gauge if and when enemies determine that a fight is not worth it. A group of bandits for example might attack and fail a morale check at the start of combat (they expected no resistance and are frightened off by PCs standing their ground), or they fail with the first casualty, or maybe after half their side is down. Or rarely they might fight to the death for their own reasons.
This gives a wide tapestry of reactions to NPCs, far more than simple violence is the only option, and far more complex than even a DM using his own judgement might provide.
