There's a reason for the double functionality, however. For example, you
can connect the standard output of one command to the standard input of
another. This is called a pipeline, and it uses the pipe operator6.1, |.

Perhaps you want to see the GNU General Public License in reverse. To do
this, you use the tac command (it's cat, only backward). Try it out:

tac /usr/doc/copyright/GPL
Unfortunately, it goes by too quickly to read. So you only get to see a
couple of paragraphs. The solution is a pipeline:

tac /usr/doc/copyright/GPL | less
This takes the standard output of tac, which is the GPL in reverse, and
sends it to the standard input of less.

You can chain as many commands together as you like. Say you have an
inexplicable desire to replace every G with Q. For this you use the
command tr G Q, like this:

tac /usr/doc/copyright/GPL | tr G Q | less
You could get the same effect using temporary files and redirection, for
example:

tac /usr/doc/copyright/GPL > tmpfile

tr G Q < tmpfile > tmpfile2

less < tmpfile2

rm tmpfile tmpfile2