During the training exercise, a cadge is useful because it...

A cadge is a specialized device used in falconry primarily to safely transport birds, and its design is particularly effective in preventing injury during transit. When falconers need to move their birds, especially over longer distances, they use a cadge to secure the bird in place. This prevents the bird from flapping its wings excessively, which can lead to injuries or stress.

Utilizing a cadge ensures that the bird is safely contained, reducing the risk of it striking against the walls of the transport vehicle or flapping around in an uncontrolled manner. Furthermore, the cadge is designed to provide adequate ventilation and visibility, crucial elements for the bird's comfort and well-being during transport.

The other options, while they may relate to aspects of falconry, do not fully capture the primary function of a cadge. For instance, while a cadge does provide some level of restraint for safety, calling it primarily a tool for safety overlooks its specific design for transport. A cadge is not intended to give the hawk free access to the area, nor is it a direct training tool, making transport and injury prevention the most accurate focus for its purpose.