You need to add a new user on your Windows server (or maybe on the domain). How would you accomplished such task?
Sure, you can fire up the GUI tool and add one user. What would you do when you need to add 2, 5, 10 or 100 users at once?
Although you can again use the GUI tools and add them one by one, the command line magic will finish the job significantly faster and without a mistake. Not to mention that you can easily automate such tasks.