In any job search, be it for a new employer or a client or customer you'll do work for, your past tells your story. Your resume, company past, what have you, says what you are capable of, who trusts you, and what you've done. If you communicate it right, people see what you're capable of – and if not, then working for them probably isn't going to be your concern.
To succeed in your career, you need to communicate the power of your past achievements, because they show you have a foundation for success. You have to show competence, skill, and knowledge in the form of results. When you show these things, and can discuss them in interviews and the like, then you convince people you're worth employing or providing services.