I initially picked this up from a branding seminar in San Francisco a few years ago, but adopted it to writing immediately. It works great in that context, because it's a question of what drives a character. Including yours.
So why do you matter? Tell me in a few words.
It's hard, right? Still, it's key to figure out your voice as a writer. You can't allow others to do it for you, because people will always try to define you on the basis of what they already know. Then you become like everyone else, and that doesn't leave a lot of room for you.
Change with change
So again, why do you matter? Still hard to answer? I understand. I have spent months figuring it out, and every time I nailed it, something in me changed, which forced me to change the answer, which changed me, which changed the answer...
That's writing in a nutshell. You find an answer, and it leads to a new question. It's almost a never ending process. Very similar to life.
Like life, however, it does end one day, and you will come up with an answer to why you matter and.
Mine is, "I matter, because I write to get a deeper understanding."