I finally found a paragraph (posted by someone else) that explains why you/your organization/your company needs one:
Do No harm: Are you prepared for an employee at your company to write on a blog that your company “sucks.” Are you prepared for a more thoughful but candid assessment by an employee? A policy should give your employees the freedom to express themselves, but parameters of what they can say and how they can say it. It should also give employers guidelines for employees who cross the line. Employees are after all employees. Everything does not go. Blogging is not permission for an employee to say anything he or she wants – even if that employee thinks it’s in the company’s best interest.
I think I need to put “create blogging policy” on my to do list for some of my friends/colleagues/clients… and myself.