There is a difference between nail polish base coat and top coat. The base coat is like a primer ready to receive a layer of paint. Its ingredients are made to help the polish adhere to the nail. Some base coats have ridge fillers added for providing a smoother application. You may notice they can be both clear or milky in color. This is because different types also provides nourishment to the nail. With the different pigments used in polishes, the base coats helps to keep the nail from being stained.
The top coat plays a different role. It creates a protective barrier on the polish to keep it from chipping. It provides that high gloss shine which helps the polish last longer. Some people choose not to use a colored nail polish, but instead, the top coat only in order to achieve a natural high shine. If you tend to mess up your freshly painted nails while wet, try finding a top coat that has quick drying features. This will help the colored nail polish dry and harden faster.