They want you to talk about how personal histories are worthwhile sources because over time they become 'distorted', which allows you to churn out even more histories of x by accounting for the personal and social forces that have led to that distortion. For example the eye-witness interview would be subject to all sorts of pressures over time, so it would enhance a written assignment because you could discuss how their version of events has come to differ from other versions, why it differs, what effect those differences have had on the person, how the event is remembered, and so on and so on.