I'm writing Java test cases and I wonder if the tests "cover" all the code. I wonder if there's a coverage tool for Eclipse Java. Five minutes later I've found and installed EclEmma and run my tests under it. At a glance I can see which lines of my code are covered (they're colored green) and which lines aren't (they're red). And I can see a summary of the percentage coverage for each of my source files. Very slick.
Coverage tools only go so far - they're not a guarantee that your tests are "complete". But they can certainly help.
Sure, there are coverage tools for C++, but there's no way I could have found, installed, and run one in five minutes.