I worked for FedEx several years ago for about 5 years, before getting into the Navy. One of the main reasons they keep such a vast fleet of business jets is because of the many hub operations they have in so many locations. A lot of times, and most of the time, business employees hitch rides on normal FedEx jets to wherever they need to go and inspect. But, sometimes a situation calls for immediate attention.
I remember a while back, probably in 2000, the FedEx system shut down temporarily because of a blackout in communication with the satellites above. One of the main locations for the eastern hemisphere's communications depot is in Paris at CDG
. Well, for a problem this large scale, several key members of FedEx had to be flown out to that location to work on the uplinks involved. This couldn't have been done as quickly as needed without a fleet of business jets standing by, primarily since FedEx's revenue is completely dependent on time and efficiency.
But anyway, with any company as large scale as FedEx is and who has so many international locations(in the thousands) it would be more problematic NOT to have a fleet of jets ready and to rely solely on commercial flights or their own cargo flights.
Hope this helps!