The simple answer is that this makes it possible to string specific command operations together as needed to manipulate or combine your data as desired. You don't necessarily have to purchase or write a specialized program for the task. The reason this works is that since everything is a file, all commmands read data from some input file, and write their results to some output file. Let's see how this works. OK, so this may not seem too useful right now.
Lets do something more reasonable.
Then, end the session with the control-D key combination. You will notice that this time what you typed didn't appear on the screen twice. Of course this isn't a very forgiving way to create a datafile since you can't edit what you've typed once you hit the return key, but it serves our purpose for this example. Now suppose that you would like to extract the second column of data from this newly created data set. When you hit the return key the numbers in the second column of your datafile will be printed to the screen.