Warm water triggers the body to start the absorption process, it is likely meant to help pull the castor oil straight through into the liver. The heat can help speed blood circulation down in there.

I don't get a flush reaction down in the intestines if I don't drink plenty of water after the salt in a salt flush, so I think there is possibly an interruption in the absorbtion process. Folk advise minimal liquid intake at meals so the stomach acid is undiluted and the digestion process goes faster- the idea is that the liquid pushes unprocessed food particles down into the intestinal tract faster.

I believe the water helps make sure the castor oil hits the intestinal tract faster.