What problem does MCP (Model Context Protocol) solve that plain tool calling doesn't, at the ecosystem level?

Without a standard, every AI app has to write custom integration code for every external tool/data source it wants to use. MCP standardizes the interface between an AI application (the 'client') and any tool/data provider (the 'server'), so a tool built once as an MCP server can be plugged into any MCP-compatible AI client without bespoke integration work each time.