Cargo One....
As you rightly say, Deutsche Post could indeed dispose of all of it's in-house flying ie DHL Air and EAT and still make money. But having recently spent $1bn on 6 new 767's straight off the production line, it's not likely. There are already a number of external carrier's operating for us but the group make vast amounts of profit from the ability to get things from one country to another in a timely fashion. Whilst we would all like a copy of 'The Big Picture', I can only assume that there is a worthy benefit to operating our own aircraft or the company woud simply use the external carrier's.
To cut a long story short, DP makes HUGE profits and largely thanks to it's airline business which, as you correctly say, is in itself a tool to facilitate the overall operation.