Comment by _bohm

In general, everything allocated within an arena has its lifetime tied to that arena. In lots of situations this is a fine or even desirable property (e.g., a single request context in a server application), but can be a tough restriction to work with in situations where you need fine-grained deallocations and possibly want to reuse freed space. The lifetime property can also be a pain to work with in multithreaded scenarios, where you might have multiple threads needing to access data stored in a single arena. Another situation that comes to mind is large long-lived allocations where you might want to have some manual defragmentation in place for performance reasons.