Why is the API key needed for bots?

Requests against the official bot API endpoints are made via JSON/HTTP and are handled by an intermediate server application that implements the MTProto protocol and uses its own API key to communicate with the MTProto servers.

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Using MTProto is the only way to communicate with the actual Telegram servers, and the main API requires developers to identify applications by means of a unique key; the bot token identifies a bot as a user and replaces the user’s phone number only.