There are many good reasons why organisations have multiple Jira instances. For many of them, this is causing more and more of a headache:
The users have to switch between different instances to track their projects. The organisations started to migrate / merge different Jira instance. They implemented automated synchronisations with different technologies. A lot of compromises had to be made, sacrificing some of the freedom of the individual instances. And it was a huge effort to get there.
With Fedration for Jira you don't have to worry any more!
Application Links
All you need are application links between your Jira instances.