Because I'm paranoid about security. My computer has only one user - me
- who has full admin rights to the box. I personally see this as a
security risk - mainly as my computer is permanently on with me logged
in. I want to separate out the admin tasks and the day to day tasks so
that, should my computer be compromised, it becomes that little bit
harder for the hacker to do any serious harm. I want to limit my access
to those few tasks I perform regularly, while reserving the more
dangerous and less frequently used commands for root access only. Yes,
I know that I have to type in my password for sudo to work, but I don't
actually see that as a serious line of defence.

And also because I simply want to learn how to do it :-)