I moderate the first post of initial commenters for the same reason I lock my doors at night. The Internet provides both darkness and anonymity, and trolls abound.
More or less, I use the same principles moderating comments on the Internet as I would at my own home: I allow respectful disagreement, but I disallow verbal abuse.
It may take me hours–or in some cases, days–to approve a comment. I spend hours unplugged, and it is glorious.