The question is really trying to get at what drives you. What are your passions, motivations and what experiences have you had to develop those passions / motivations. To the extent that your professional experiences have helped define your passions / motivations, they should be mentioned. The same goes for your personal experiences. So, for different applicants the exact answer to your question will be different.
wow! this really gave me some interesting ideas. this is good, truthringer. i liked your last statement