代码覆盖率的真正目的

In a sentence: Code coverage tells you what you definitely haven't tested, not what youhave.

Part of building a valuable unit test suite is finding the most important, high-risk code and asking hard questions of it. You want to make sure the tough stuff works as a priority. Coverage figures have no notion of the 'importance' of code, nor the quality
of tests.

In my experience, many of the most important tests you will ever write are the tests that barely add any coverage at all (edge cases that add a few extra % here and there, but find loads of bugs).

The problem with setting hard and (potentially counter-productive) coverage targets is that developers may have to start bending over backwards to test their code. There's making code testable, and then there's just torture. If you hit 100% coverage with
great tests then that's fantastic, but in most situations the extra effort is just not worth it.

Furthermore, people start obsessing/fiddling with numbers rather than focussing on the quality of the tests. I've seen badly written tests that have 90+% coverage, just as I've seen excellent tests that only have 60-70% coverage.

Again, I tend to look at coverage as an indicator of what definitely hasn't been tested.