To have good test coverage you should both test all possible inputs and have proper asserts for those inputs.
Testing is hard, covering lines of code isn't. To put it another way: in a test the hard bit is the assert, not the call into the tested code. And coverage only reports on the former.
If a e.g game contains a sorting in some place in the renderer, I can replace the quicksort with a mergesort as long as the renderer interface is still testing ok. The new sort algorithm may have new special case paths (even number of items vs odd for example) but it's not a concern of the renderer public interface. I may however have introduced a bug with an odd number of items here and the old code was 100% covered and now it isn't. So there is a potential problem and the 99% has actually helped spot it.
If the sorting is a private implementation detail of the renderer then there is no other place to test it than to add a new test to the renderer component only because the sorting algo requires it for a code path. This is BAD.
The proper action here is NOT to add tests to the renderer component to test the sorting code path, but instead to make the sorting visible and testable in isolation via its own public interface.
So one of the positive things about requiring coverage is that if you do it right, it will lead to smaller and more decoupled modules of code.
The bad thing is that if you do it wrong you will have your God classes and a bunch of tests coupled tightly to them.
At the start of your function, check all the prerequisites, e.g:
if(x<0) throw "x should be non-negative. Got $x"
if(x>=n)throw "x should be smaller than n. Got $x and $n"
if(x=0) edgeConditionsHit += 1
if(x=n) edgeConditionsHit += 1
If that doesn't hit 100% in the rest of the function, the function has code it doesn't need, or your precondition checks aren't complete.
Think about other edge conditions. For example, does your code special-case x=n/2? Add a check on top. And yes, that is implementation-specific, but there is nothing you can do about that.
Of course, you don't need the edge condition and implementation-specific checks in release builds.
With these in hand, you can also split tests into implementation-specific ones and contract-based ones.
Of course, this still isn't a guarantee. But it does make me feel better about my code.
So yeah, when a team claims 100% code coverage, usually that is just a signal that they care about testing and the quality of the code, therefore it tends to be less buggy. Not necessarily because 100% coverage itself made it so.
Really I only use a code coverage tool to check for important places that aren't covered at all, AFTER I have tried to think of the proper behavior/spec of the code from an outside perspective. It's like a secondary check after you think you are already done. That keeps you focused on what correct input and output are, and then patching up the little areas that you missed with a tool.
»Our customers tend to be makers of aircraft or car
parts. Both businesses have strict safety standards
which involve coverage testing, and our tools help you
produce the relevant reports for certification, like
DO-178B for the aviation industry.«
And yes, it can. Are you trying to state their value compared to what? A good schema for task division, with encapsulation and whatchdogs? Proofs of correctness? Proofs of halting? Testing is much less valuable than any of those.