From a (very) high level perspective it allows you to use Oracle SLA and GL to perform the accounting for third party applications. Companies the Financial services industry tend to build a lot of highly complex applications (e.g. loan systems, trading systems) in house, but they want the final accounting of transactions form these disparate system to end up in the same place and it will be nice if they can re-use the same accounting rules too. Hence the concept of an accounting hub.
If you are using FSAH on R12, you can also utilize all the new AGIS features introduced in R12 and that could help you to apply common intercompany processing rules across your entire business too.
I have had a few discussions with customers asking if AGIS could be used as a hub for all intercompany trading across many ERP systems and the answer is very much so, that is something that we had in mind when building the product. You should be able to push transactions from multiple Oracle instances, even on different versions and throw in an SAP or Peoplesoft instance too if you want. There will be some integration work, but we use standard interfaces, services, API and also make use of the Workflow Business Event system to make this process as easy as possible.
UPDATE(April 27, 2009): New post with links to more information on the Accounting Hub.