Zones are a way to segregate objects. This is useful for a few reasons:
an easy way to clean up
If you run a computation for some set of parameters and then want to clean the slate to run another, you just drop the Zone where you put all the components of the computation, and every component will be discarded.
to monitor a computation
For example, if you have a simulation of ten communities, and each member of a community is in that community's respective Zone, then you can easily monitor the growth and membership of a community by asking that community's Zone to describe itself.
locality of reference
The main thing that slows down a modern computer is memory access. When the CPU has to look outside of its registers for data, there is a penalty to go to the cache. When the cache doesn't have what is requested, then virtual memory must be accessed, which is a very large penalty. When the virtual memory doesn't map to real memory, then there is a massive penalty. The closer data is kept to a computation the faster the process will be.