The truth is when you copy file of a directory by ADD it acts exactly as COPY do. If you use ADD with tarball archive, it will extract it automatically into the directory, which is supplied as a second argument.

Example:

ADD file.tar.gz /code

is exactly the same as

COPY    file.tar.gz /code/
RUN     tar -xvzf /code/file.tar.gz

and will save you one line