Well, its pretty simple: Say say 20k miles get you from London to Dublin; but with the same 20k miles you could have flown till Istanbul; so distance wise you got much more out of your miles when traveling to Istanbul. (money wise it might not pay, of the London-Dublin route for some reason is very expensive when buying)

Now using your brains, you will probably find loopholes or ways to go very far using the same amount of miles which were intended for much shorter flights.

My example was: traveling from London to Seoul for 4500 miles in Business Class, which is unbelievable.