To make the drum easier to put back on.
When you have fitted the shoes onto the backplate and fitted the shoe anchor pins, and the springs. ie it is fully assembled.
Tap the shoes inwards towards the centre stub. they will move inwards on the tight adjusting springs allowing you to put the drum on easily.
When the drum is back on and fixed properly, press the brake pedal a few times to recentralise the shoes in the drums.
If you still don't have enough pull on the handbrake ie. the nut is as far as it will go on the thread, get a piece of steel tube to go over the adjusting screw, of course after taking the adjusting nut off and then put the nut back on.
This will give you more adjustment
Dave