For set bj_wantDestroyGroup = true
is when you want to destroy Unit Group that is not set via variable.
For this one, the default setting of the boolean is equal to false, which means the Unit Group function never clean the leak caused by this function.
The function checks the boolean for it to be destroyed or not: true = destroy | false = does not destroy
So by setting it to true, it will destroy the Unit Group once after it's been used.
When you want to destroy Unit Group that is from a variable, do this: call DestroyGroup(udg_variableName)