The cornucopia is used to exchange magic weapons with the psychic in the small ring. It is when the magic weapon is refined by blood that the result of blood refining can be guaranteed to be the main magic weapon. Each magic weapon has its own corresponding media, which can basically be exchanged with a cornucopia.
After a higher level, you need blood to make a good magic weapon. The cornucopia is indispensable, but there are many things sold in the first line, ranging from a few gold to more than a dozen gold.