Ignoring for a moment all of the other functions that MCP can allow an agent to do (open a webpage, query a database, run another agent, execute local commands etc) and only focussing on the use of MCP to provide context, the big advantage of MCP over RAG is that a RAG system needs to be built and maintained: you need to extract your content, vectorise it, store it in a database, query it, update it etc etc. With MCP, you just point it at your database and the agent gets up-to-date info.