You're all correct about the use, but if you look at KChen's life drawings, read his posts and look at the pic of his tools, he clearly uses the razorblade to sharpen the conte and pencil. And the original question was what KChen uses the razor for. And now, I believe that this part of the topic is quite dead. It is an ex-topic, it ceases to be. It is no more.
Now, back to posting how amazing KChen is until he posts more amazing drawings!!