Two main advantages:
1. As G4G5 said, it raises badly-needed cash for American.
2. If Arkia is profitable, it allows them--as the owner of the planes--to take depreciation and reduce their net income for tax purposes. American does not need this tax benefit because it is not profitable.
If you read between the lines, you might notice that this sounds an awful lot like a loan, with the aircraft as collateral. However, there are important differences...that's where AA's excellent legal department comes in handy.