mmap is not only to map a file content into a process address space, it can also be used to request memory from the OS using the MAP_ANONYMOUS flag. Early on a malloc-like allocator would use brk/sbrk to get a chunk of "raw" memory, and manage this for the user providing access using malloc/free. Nowadays the same "raw" memory can be obtained using an anonymous map. A raw pool, when unused, can be returned to the OS by unmapping it.

Using an anonymous map provides address space randomization by default (for the raw pool location), whereas brk/sbrk extends a limit and because of this are predictable.