Cheaper and faster retrieval to be added to the context and discoverable by the agent.

You need more git commands to find the right commit that contains the context you want (either you the human or the LLM burning too many token and time) than just include the right MD file or use grep with proper keywords.

Moreover you could need multiple commits to get the full context, while if you ask the LLM to keep the MD file up to date, you have everything together.

I doubt you can give more context to an LLM from a README file than 500 properly written commits. Or to a human for that matter.