purpose of white box testing

I have read about whitebox testing. Statement coverage, branch coverage, path coverage done at unit, integration levels. In this we derive test cases to obtain maximum coverage for statements, branches and paths. But I want to ask what we do with test cases built during white box testing? I mean in black box testing we execute test cases on application under test in order to check them whether they are passed or fail. But what we do with the test cases of white box testing?

My second question is what type of defects are found in white box testing?