FDX1311 is a daily scheduled flight, called a flying spare, or more colloquially a sweeper. It's a provision to remedy unplanned events which would otherwise prevent customers shipments to reach FedEx Memphis super-hub before dawn, and eventually to prevent shipments to be delivered in time.
Fedex sweep flight 1311, Youtube
Route and reuse after landing
The flightpath is designed to allow the aircraft to be diverted to several airports in the US south area where FedEx has an important activity. When reaching Memphis airport, the aircraft becomes available for dispatching shipments from the hub.
FDX1311 is part of FedEx contingency plan which, according to Fortune, comprises five aircraft to cover the whole US territory (to be compared to the 600+ aircraft fleet).
Diversion is triggered in quite different cases:
To fix unplanned maintenance. Aircraft availability and safety involves preventive maintenance and unavailability is well scheduled. But unplanned failures or damages still occur, some shipments can be left without a carrier. The spare aircraft will fix this problem.
To fix volume prediction errors. The number of containers can be predicted as soon as the shipments are collected, and aircraft can be dispatched accordingly for the next 24 hours. However there can be mistakes. Mistakes involving large volumes/weights cannot always be fixed with the planned aircraft. Containers in excess may be left at the warehouse. They will be collected by the spare flight.
To collect forgotten shipments. Errors may occur during containers preparation or loading into the holds, resulting in customers shipments forgotten or wrongly dispatched. The spare aircraft will get take them to Memphis in time.
According to FedEx each flight costs $30k. Most of the time it just flies this route empty or lightly loaded. Fuel consumption is about 30% higher than for the direct path.
Individual customer shipments may experience delivery delays without large consequences, or without strong reaction. This is not the case for large customers for which contracts definitely include a delivery delay commitment, associated penalties and contract termination clauses for service failures.
I guess this $30k a day investment for this region is the quality assurance response to the evaluation of risks above, and those I'm not aware of. This is good for image, for competitiveness and for contracts renewal.