When you're talking about yourself, your brand or your organization, you may have first-person credibility... but you also have a pretty obvious conflict of interest. Add that to the growing distrust of advertising and public relations - in fact, of institutional communications generally - and you have a challenge.
These days, your audience is putting much more trust in their personal networks: their friends, family, neighbours and colleagues. When they hear a personal message from someone they know, it punches through in a way that organizational communications can't.