Grease has the potential to buildup along fan components, most prominently, the fan blades. When this occurs, the fan’s balance is disrupted. Within a short time, the fans belts and bearings will wear out prematurely. The safety and effectiveness of your kitchens exhaust fan system drops to zero not long after.
For these reasons (and many other), the local codes of most municipalities require the installation of paneling that facilities easy access for routine cleaning and inspection of your kitchens exhaust fan system.