Leaders aren't always managers and vise versa. It is a rare individual that is both of these things. They have very different skill sets, both critical to success at a high-growth business.
Understanding who your leaders are and who your managers are will help you create an organizational structure that not only addresses core business functions and needs but also morale and culture, which are equally if not more important. It will also help you identify where there might be gaps or people in the wrong "seats on the bus," to quote Jim Collins.
Leaders have a unique ability to rally employees around a vision. Because their belief in the vision is so strong, employees will naturally want to follow them. Leaders also tend to be willing to take risks in pursuit of the vision.
Managers, on the other hand, are more adept at executing the vision in a very systemic way and directing employees on how to do so. They can see all of the intricate moving parts and understand how to make them harmonize. Managers are usually very risk-adverse.
Deep down, a lot of entrepreneurs are leaders and not managers. I'm one of those. I don't think that I manage well, and if I had to focus solely on that it would be extremely painful for all involved. Conversely, if a manager is expected to lead a company, that company will be managed into a nice, tidy grave.
It's true that some managers can inspire and some leaders can systemically execute, but these are not their core strengths. For a start-up, the entrepreneur really has no choice but to be both leader and manager, which is usually okay since it's probably just him/her and one or two others. Understanding which you are will help you make important, early choices about whom you need to grow that complement your strengths and ensure the success of your business.