They mention using this as the backing array for a power-of-two-sized hash table, but I don't think it would be very useful considering that the hash table won't provide stable pointers, given that you would need to rehash every element as the table grows. Even if you just wanted to reuse the memory, rehashing in-place would be a PITA.

I think they mentioned it's for an arena, where stability is necessary. You might happen to use said arena for a hash table