Gary Vaynerchuk could tell you that his personal brand is worth millions, but he's modest. My friend and PodCamp cofounder, Christopher S. Penn, often refers to branding by ZeFrank's definition: "an emotional aftertaste" (see the The Show with ZeFrank episode). I have some thoughts on how you might develop a strong personal brand online, and what you might do with one after you build it.
You might already know the answer to this question. There are lots of answers, actually, depending on you, your needs, the way the world has shaped you. Let's look at just one answer.
The easiest answer is that you might want to be memorable, and you might want to transfer your real-world reputation to the online world. A strong personal brand is a mix of reputation, trust, attention, and execution. You might want to build a brand around being helpful (what I hope my brand means to you), or being a creative thinker (e.g., Hugh Macleod) or being a deal maker (Donald Trump), or being a showman (David Lee Roth), or whatever matters most to you—and what you are capable of sustaining.
A personal brand gives you the ability to stand out in a sea of similar products. In essence, you're marketing yourself as something different than the rest of the pack. Do you need this? I don't know. Do you like to be mixed in with the pack?
What's the difference between Coke and Pepsi? There's a taste difference, ...