Having the prototype available before the first use of the function allows the compiler to check that the correct number and type of arguments are used in the function call and that the returned value, if any, is being used reasonably. The function definition itself can act as an implicit function declaration.
While shellac is not usually the finish paint choice, it is often thought of as the best paint primer available. It has properties that oil and water based paints can't match, and provides a more dependable seal than most other primers are able to provide.
Last but not least, the question of why does shellac or gel nail polish last longer? must be resolved. Let's start with its ingredients. Shellac nail polish isn't made with nitrocellulose like regular nail polish is; it's made with methacrylate monomers (something you'd find in acrylic nail polish).