You're confusing the agent and the LLM.

An LLM doesn't "use" anything. Your agent does that. The agent is the program that prompts the LLM and reads the response. (The agent typically runs locally on your device, the LLM is typically on a server somewhere and accessed through an API.)

For the agent to parse an LLM's reply properly you'd ideally want the LLM to give a response that adheres to a standard format. Hence grammars.

I'm guessing your confusion stems from the fact that you've only ever used LLM's in a chat box on a website. This is OpenAI's business model, but not how LLM's will be used when the technology eventually matures.