when do you need to use a power distribution block in your schematic?
As far as I know, you don't need a power distribution block in systems which has more than one 3-phase motor. A power distribution block helps you to create more efficient wiring. 

When you use a distribution block, it is no longer necessary to switch every device in the control circuit back to the power source. This can be done with the help of one large wire on the input side and several smaller wires on the output to the downstream devices. All this makes the whole look more professional and better.