What's the state message useful for in a HTTP response message?

Ramy Al Zuhouri asked:

My book makes an example where the first line of the HTTP response is:

HTTP/1.1 200 OK

Then there is a table where explains all the codes, and 200 is equal to ‘OK’.

Then what is the ‘OK’ useful for, and why is there this redundancy?

My answer:

It means exactly what it says in RFC 2616, which you should have read:

6.1.1 Status Code and Reason Phrase

The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. These codes are fully defined in section 10. The Reason-Phrase is intended to give a short textual description of the Status-Code. The Status-Code is intended for use by automata and the Reason-Phrase is intended for the human user. The client is not required to examine or display the Reason-Phrase.

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