Can anyone explain to me what is the purpose for creating different farm and zones in Citrix ?

Hi guys, I know how to configure Citrix servers in a single farm and single zone. May I know when would I ever need to configure a seperate farm or zone as a single farm and zone is enough? Ok let's start with farm. When do I need to configure 2 seperate farm? Say Farm A in Singapore and Farm B in Australia and are connected via a WAN link. What are the true benefits of configuring seperate farms? Do we create seperate farms if they're hosting the same or different applications? In the citrix management console? Do I see both farms and can manage both farms?

How about zones? I know servers in a single sone will keep most network traffic among citrix servers in a zone to themselves. When then we need seperate zones? Say in a single company, do we need to create seperate zones for servers?

Please help as I'm confused :)