The valley of fear

If I located the correct section, the need for two notes can be found in the fact that Porlock was afraid for his life.

“Dear Mr. Holmes [he says]:
“I will go no further in this matter. It
is too dangerous—he suspects me. I can
see that he suspects me. He came to
me quite unexpectedly after I had actually addressed this envelope with the
intention of sending you the key to the
cipher. I was able to cover it up. If he
had seen it, it would have gone hard
with me. But I read suspicion in his
eyes. Please burn the cipher message,
which can now be of no use to you.
— “Fred Porlock.”