One reason is to make the olive oil easier to drink. Dr. Clark says that only fresh squeezed PINK grapefruit will do that.
It says in her book "Close the jar tightly with the lid and shake hard until watery (only fresh grapefruit juice does this)."
It may also have some effect on how the liver and gallbladder react to the olive oil but I am not 100% sure of that.
Plus it makes the large amount of olive oil that you are going to drink taste better.