Comment by flohofwoe

The actual point to understand is that a general purpose allocator is usually "mostly crap" because it requires an incredible internal complexity to meet all requirements. It's often better to use specialized local allocators, and write code which expects an allocator to be provided instead of calling into global allocator functions. Such specialized allocators can often be a lot simpler, while being at least as fast general purpose allocators like jemalloc or mimalloc.

Very often you don't actually need to track or manage the lifetime of individual objects, since related objects are often created and destroyed at the same time. For instance when parsing a JSON file you might end up with many individual nodes which can all be discarded at the same time once the parsing result has been consumed. With an arena allocator, you just throw away the memory for all those nodes at once, instead of calling a free/deallocate/delete functions tens- or hundreds-of-thousand times.