The recent move by Facebook to allow the selection of usernames has prompted some to ask: why? The answer is personal branding.
Your online presence is part of your brand. I would argue that it is a very significant part. How much you care about or publicize that brand depends on a number of factors. One thing to note, however, is that a quality brand must be built up over time. In other words, it is easier to gradually build up and maintain your online presence than to find yourself needing one and thinking "hmmm, maybe I'll try the internet now."
Why would you need one? Well, I found my current job (it found me) through my online presence (facebook). You can make money with a strong enough brand through blogging or online content. Your online brand can help grow your online social network beyond your physical social network.
Getting a username is like having a professional logo for a business. A business could just write their name in Helvetica and be done with it. The same information gets conveyed, and people can find them. However, a nice logo grabs people's attention, and a nice sign will make it easier to find while driving past it.
Think about it in terms of trademark. Why do companies fight so hard to keep their trademark? In our information age the ability to reach down to individuals has multiplied exponentially. Do you want somebody infringing on your personal trademark? What about if that infringement affects your future job search? Your spouses future job search? Your children's future job search?
Here are some additional places to look on wikipedia: a blog post about it. Only the paradigm shift is that you are the content!