It's obviously possible to find niche cases where the standard containers are not good enough. But the remaining 9999 cases out of 10000 are more interesting.
I also had one of those, in an application that created and deleted a large number of fixed-size arrays across many threads. A naive implementation using glibc malloc ended up with a massive memory leak caused by hundreds/thousands of fragmented arenas. A thread could usually not reuse the memory it had just freed, because it was using a different arena. And because the arena was not empty, it was not possible to unmap the memory.
I also had one of those, in an application that created and deleted a large number of fixed-size arrays across many threads. A naive implementation using glibc malloc ended up with a massive memory leak caused by hundreds/thousands of fragmented arenas. A thread could usually not reuse the memory it had just freed, because it was using a different arena. And because the arena was not empty, it was not possible to unmap the memory.