air in the system
If you think about it your coolant system is a bunch of channels and tunnels that go through the block. The more turbulent the flow the better the heat transfers so the coolant has to go through little turns and twists quite a bit. If you don't drain the block and the rad seperately then you would have coolant and air pockets in one or both.
If you are just changing the WP then you can get away without draining the block completely but why not flush it all out since you in there anyway. I actually ran a garden hose wide open where the WP and thermostat meet the block to make sure and move all the old coolant out when I changed my WP.