SRM Purchase Organization

Hello everyone,

I have a question regarding the Purchasing Organization in SRM. We create 2 purchase organizations in the SRM org structure, one with link it to the backend system and create another as Local purchase org. what I interested in understanding is why do we need a local purchase org? what is the objective of having local purchase org when we have the back end Org linked to the SRM org structure? And why is it that the Shopping carts use the local purchase org and local Purchasing groups? Can anybody throw some light on this?

Thanks

Rashmi