Because it has been so firmly established for so many years, appropriate service and loading/unloading infrastructure is commonly available at airports capable of landing the 727. However, the 727 features an Auxiliary Power Unit (APU) which permits operations from smaller or more remote airfields which lack ground equipment.
For many years the 727 was the world's most successful airliner, one of the most graceful aircraft ever made and with its T-tail and tri-jet it is still one of the most recognisable.
It was the backbone and principal workhorse of domestic airlines around the world. Over 1,800 727s were produced, a number only ever surpassed by the 737.
Now modernized, it is expected to serve for at least another 20 years in freighter configuration.
In fact, from the earliest days the 727 was built with freight-lifting in mind, as can be seen by the size of the large main cargo door.
The 727 was built as a short-to-medium-range jet, and was designed to be able to operate out of small airports with shorter runways.
The three-engine design was chosen to give the aircraft an edge in performance over two-engined aircraft and still be more economical than four engines.
The 727 has three cargo holds. The main (upper) hold can carry a variety of loads including palletised cargo in custom-fitted containers and it can even be fitted to carry up to 16 racehorses.
The two belly holds can carry any manner of volumetric cargo.
The 727's only real rival for capacity and load is the Hercules. However, the Hercules, of course, is limited to military/relief work, it is very much slower and its range is quite limited compared to the 727.
With its range/payload curve the 727 is the ideal solution for rapidly lifting medium-sized loads:
the Hercules is not an option, is not available, is too slow, or