This has been bugging me for a while. I'm very interested in Shisui's story of his life, but the real question is why was Donzo after his eye? Was it because of his sharingan? and "how is Donzo supposed to use this"?
Shisui was one of the few members in the clan's history that was able to awaken the Mangekyō Sharingan. It granted him access to a very powerful genjutsu, a unique technique noted to be of the highest caliber, Kotomatsukami. This technique allowed him to enter a person's mind and manipulate him or her by creating false experiences and illusions making it seem as though they were doing things of their own free will. The victim wouldn't be able to notice that they were being manipulated.
This Mangekyō Sharingan are noted to be the "heavenly eyes that see the truth of creation without obstruction". Hence Danzo was after Shisou's eyes.Source