Rotary cutters are very useful for materials that tend to buckle or bend away from ordinary knife blades. When using a circular knife most of the pressure is directed straight downwards. There is nowhere for the material to escape.
The amount of protection provided for the blade, when not in use, varies between manufacturers. They typically use press-out mechanisms, which can leave a small amount of the unprotected blade when it is in the closed position, or they have a shield, which can be fiddly to open and shut.