There are some good reasons to use this. A typical thing is to have a descriptor header followed by multiple types of data. The descriptor header says what kind

There are some good reasons to use this. A typical thing is to have a descriptor header followed by multiple types of data. The descriptor header says what kind of data is following. You can obviously just use a void pointer in the descriptor that points to a separate data struct, but having them both in the same memory block means fewer allocations, and fewer cache misses, giving better performance.