Out of own experience doing several buyer persona projects for clients the key insights come from interviews with (non-) clients. Just for the simple reason they are the truth concerning your product or service. If you do not incorporate interviews, your buyer personas will be based on the internal truth of your company and not on what the real questions, needs and goals of your buyers are. Often is this different from what you think. And these key insights make the difference in creating successful (gated) content to support your buyer in his or her journey.

As a marketeer knowing and talking to your clients is key to understand them and be relevant for them. Strangely enough most marketeers forget this and use the internal truth.