Typically you don't need it, persistence contexts are always provided for you.
However, if you wanted another persistence context, you can always get hold of the factory and create one. For example, if you had an entity, and you wanted to see what was in the database, you could use a second persistence context to view the currently persisted data.
You could also use it to query data if you wanted to get around the fact that changes to an entity pool are flushed when you run a query. Running the query on a second entity manager would sneak around that.
Thanks for your reply!