Advantages of White-box Testing
An advantage of white-box testing is that it is thorough and focuses on the produced code. since there is knowledge of the internal structure or logic, errors or deliberate mischief on the part of a programmer have a higher probability of being detected. [Source]
Using white-box testing methods, the software engineer can derive test cases that (1) guarantee that all independent paths within a module have been exercised at least once, (2) exercise all logical decisions on their true and false sides, (3) execute all loops at their boundaries and within their operational bounds, and (4) exercise internal data structures to ensure their validity. [Roger S. Pressman]
Reveals error in the hidden code.
Also See:
White-box Testing
Limitations or Disadvantages of White-box Testing