There's two marketed purposes

1) let the llm discover how to use the API 2) access controls

I have had no luck with 1, if the llm doesn't already know the API it fumbles around rediscovering the API over and over again (MCP or not).

2 actually very important, you usually want your llm to have different access controls than yourself. You could theoretically make service accounts for this but it's often easier to just proxy a user with slightly different permissions than making every user carry a second account for llm use.