I assume that you are asking about the letter that Dr. Jekyll gives to Mr. Utterson in Chapter 5, "The Incident of the Letter."
If that is what you are asking, the answer seems to be that Jekyll wants his lawyer to know or to believe that Mr. Hyde will not be coming around Jekyll anymore. Utterson has been worried about Hyde's influence because of what is in Jekyll's will.
By showing Utterson this letter, Jekyll hopes to make him feel better because the letter says Hyde is never coming back. That way he will never influence Jekyll again.
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