Our bodies do a fine job of digesting plant material, thus the popularity of vegetable sections and salad bars in grocery stores. However by diffusing the chemical properties of MJ into a lipid (oil/butter) your body has fewer steps to go through to get the thc in a condition that will allow it to enter the bloodstream.
Raw plant material has to first be broken down before it's chemical properties can be absorbed, depending on how long this takes the absorption of thc may occur at such a slow rate that the desired effects can be lessened or non existent. With fat based thc the need to breakdown the plant material is skipped and fats pass readily into the bloodstream.