"Do it, Harry" is a simple imperative -- telling someone what to do. "You are to do it, Harry" is a more "complicated" statement, not strictly an imperative, with connotations that it has been decided/planned/ordered that Harry will do it.
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"Do it, Harry" is a simple imperative -- telling someone what to do. "You are to do it, Harry" is a more "complicated" statement, not strictly an imperative, with connotations that it has been decided/planned/ordered that Harry will do it.