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.
They re-labled a 2# cut of shellac as a sanding sealer for use under oil based poly and waterborne finishes. As a initial coat between sanded, virgin wood and any other top coat, shellac offers no benefit. There is an exception with using an oil based or waterborne finish over an tropical, oily wood.