Companies also track and report is when required by external entities such as the FAA for safety critical systems.
I think it goes deeper than "the conditions you forgot to write" although that's part of it. A developer's test suite tends to make assumptions and these are the same assumptions that exist in the production code. If your assumptions about usage, or input, or edge cases are poor then the tests you write will be poor too and in that case 100% coverage isn't worth much. Big reason I'm a fan of things like QuickCheck in Haskell.
I think it goes deeper than "the conditions you forgot to write" although that's part of it. A developer's test suite tends to make assumptions and these are the same assumptions that exist in the production code. If your assumptions about usage, or input, or edge cases are poor then the tests you write will be poor too and in that case 100% coverage isn't worth much. Big reason I'm a fan of things like QuickCheck in Haskell.