For me (actually trying to get shit done using this stuff) it's validation.

Being able to have a verifiable input/output structure is key. I suppose you can do that with a regular http api call (json) but where do you document the openapi/schema stuff? Oh yeah...something like mcp.

I agree that mcp isn't as refined as it should be, but when used properly it's better than having it burn thru tokens by scraping around web content.