Aside from the obvious reasons...
I am a pilot for Atlas Air Cargo and we have a crew base in Europe.
I have been based in Europe since July and have been commuting to work from PA.
I had to train for 2 months earlier in the year to fly a newer version of the 747.
New Airplane 747-400
From first impressions you might think, "why would he have to train for 2 months on an airplane that looks the same?".
Old Aiplane 747-200
The flight deck(cockpit) of the aircraft are completely different. We had to fly the old airplane with 3 people. And due to computers and techno advances we only need 2 pilots for the -400.
Some of our airplanes are painted in our customers colors. I'll frequently be flying for Emirates, into and out of Dubai.