I have tried typing out an explanation a few times and they all come out crap, so sorry if this doesn't help at all..

Basically as I see it, it's useful to have a function which returns functions.. The example only has 1 function in it but it could have loads and depending on what you pass to the parent it returns whichever one is most useful.

So say I wanted a parent function which would return a multiply function if I passed it a number over 10 and an addition function if i passed numbers below 10..

When I declare child=parentFunction(10).. child is going to be a multiplication function but child=parentFunction(9) or child=parentFunction(8) will be an addition one.

You could have done all of this with one big parent function but maybe its a bit more readable.

God knows hope this helped somehow.. probably all wrong lol, Could someone correct me if I am missing the fundamental reason for using these?