It is to prevent hydraulic lock due to oil getting in the cylinder.
This can cause a bent con rod as the oil prevents the piston from getting to TDC.
This then allows the bottom rings to exit the bottom of the cylinder and then they break off on their way back up. So you end up with a bent conrod, a damaged piston and cylinder and the engine full of bits of piston ring.
This is a lot more expensive than pulling the prop through.