First and foremost, suppressions can be used for more than leaks. For example, for "Conditional jump or move depends on uninitialised value(s)", which yes, there are very much false positives for because of e.g. tricky optimizations LLVM performs and that valgrind can't handle.

But even for leaks, you can also have intentional leaks that valgrind will flag but that you can't really do anything about. One example is how using `putenv` can lead to you having to leak memory on purpose. There are many other cases.