The gel coat on a GRP boat over time will oxidize, stain and lose its gloss. An acid wash when she is on the slip or hardstand is a cheap and easy way to remove the oxidization, brighten the colour and remove stains. It will not restore any shine whatsoever, for that you will need to have the vessel machine polished.
All boats from time to time can get a brown "moustache" where the water pushed up from the bow wave is constantly washing past the hull. The acid wash process will remove this stain without scratching or causing any damage to the gel coat. Your boat will look much cleaner and brighter.