Ever heard a statement like, only the developer knows the code? As much as coding and development of software with the same language may have lots of similarities, individuals differ and their style of approach as well.
An application may not function as it ought to but could be maneuvered by the developer to appear as though it is working well. A developer could also show false claims of functionality of a software he designed, hence this software is best tested by experts who grade and ascertain the app's quality based on numerous criteria.
