Knowing yourself matters for all sorts of reasons: general happiness, success, and avoiding being perceived as a jerk. Personal branding matters because it’s how you let the world know who you are. That said, if you are the aforementioned a-hole, maybe you should work on that before you let the rest of the world meet you.
Chris Brogan (not an a-hole) has a great, free, and short eBook on Personal Branding you can download here. Good stuff.