Low-fidelity prototypes help enable early visualization of alternative design solutions, which allows provoking innovation and improvement. They are simple and produced very quickly to test the broader concept, e.g. Paper sketches.
High-fidelity prototypes typically use realistic (mouse-keyboard) user interactions, and are much closer to how a user would usually navigate your design. It creates a close likeness possible to an accurate representation of the user interface. They are more effective in accumulating accurate human performance data (e.g., time to complete a task), and in demonstrating actual potential applications to clients, management, and others.