Everything we put online or on our book jacket is part of our marketing effort. Anne R. Allen asks if your author bio helps your book sales or stops them dead, and Lisa Tener shows how to get bestselling authors to write blurbs, forewords, and Amazon reviews.
Who SHOULD write it: the author, with some editing from marketing. Actually, even though I just described how I'm an author who wrote my own, I'm not sure I agree that it's always best for the author to write it. Authors, I can say from both firsthand and secondhand experience, tend not to have a great deal of perspective on their own work.