In many workflows LLMs will violate semantic guidelines, thus preventing them from acomplishing their task. For example, agents will sometimes forget the shape of their tool calls, mistake the type of an in-context variable, or perform unauthorized operations on data.
AUFBAU builds, token by token, continuations that keep a partial syntax tree well-formed and well-typed according to user-definable attributes on a context free surface. These attributes are generic, from type constraints to relational rules between objects.
In short, you can give hard rules your LLMs will always follow.