The two truths keep the tradition from talking nonsense in either direction. Conventionally there are people, promises, and consequences, and Buddhist ethics operates entirely at that level; ultimately those are designations laid over processes with no fixed cores. Both statements are true at their own level, and the error is collapsing them — either denying that anything matters because it is all empty, or insisting that conventional things must therefore be ultimately real. Every school has a version of the distinction; they disagree sharply about what the ultimate truth actually is, and whether it can be stated at all.