Perfective verbs tell you about a finished action, eg wypić informs you that you drank everything, there's nothing more to do. Imperfective verbs don't contain that information, it's most often an unfinished action. When you say pić you don't know whether the person has finished or still has to complete it. It makes more sense with verbs like malować - namalować, śpiewać - zaśpiewać etc.