Simply put, on-camera (or 'axial') flash results in flatly-lit, ordinary images that are rarely different from what one would get with a point & shoot camera. Photography is all about the quality, direction and focus of the light used to illuminate the subject matter - good quality lighting is not achieved by chance, it's by the study of its properties and what makes flattering, detailed and extraordinary images.
Along with Planet Neil's flash photography tutorials, also study
Strobist's Lighting 101 and 102 tutorials - they're quite an authoritative set of lessons and step-by-step instructions on getting the right lighting for the subject you want to shoot.