It is pure fun.
Some use it to try different tactics.
Some try to test players on different positions.
I mostly use private league to test my team against Friends who normally play in a different league. And as well testing my team against the WORLD. So therefore I invited a lot of teams across to world to play in a kind of WORLD LEAGUE.
Its fun; and no training minutes or experience or flags are accumulated.
yesterday is history, tomorrow is a mystery, but today is a gift. That is why it is called the "present."