You can allocate a part of a arena to form a new arena, at least theoretically.

And in many cases for compliance you want to 1) limit the memory a specific sub-component of your code can use 2) make sure it always can have this memory 3) make sure it doesn't leak any memory ever. Arenas are probably the most reliable way to archive this.

The main problem is if the sub-component needs to allocate memory which outlives it's life time. A common way around this is that the component delegating work to it has to provide that memory (e.g. windows kernel API) but that doesn't always scale well. Another is to use another memory management for it (e.g. GC) but then it isn't enforcing compliance anymore. Etc. it's always some compromis.