An LLC requires Managers to run it. Basically, they are like the Board of Directors for a corporation. The rational behind splitting up management of the Company from Ownership is to allow for passive owners for a company - people who just want to put in money and own a piece of the pie, but don't want to get caught up in the day to day management and operations.
As your Company grows, you may want to appoint Officers even though it is not required for an LLC. As the Board of Managers steps back and begins to make high-level strategic decisions, it can be helpful to have officers take over the actual management of the Company. It can also help distribute responsibilities among the founders. For instance, one founder can be CEO, another CTO (chief technology officer), another CMO (chief marketing officer) etc.