No! Constant f/2.8 aperture lenses maintain a maximum aperture of f/2.8 throughout the entire zoom range of the lens. With Variable aperture lenses like the 70-300mm f.4.5-5.6G the maximum aperture varies from f/4.5 at the wide end to f/5.6 at the long end. This refers to the maximum (Largest Opening) aperture only. You can always stop the aperture down (close) on either type lens.
Fast lenses allow you to use a faster shutter speed in low light situations providing the ability of freezing the subjects motion. Shooting wide open also provides a narrow DOF resulting in subject isolation. Good Luck and Enjoy your Nikons!