Why were Kievan Russia and the Byzantine Empire important to each other?

Part of the importance relied in the facts that the Byzantine Empire introduced Christianity to the region, and a separate Russian Orthodox Church soon became more popular. Besides, a formal law code was developed, and state courts were created. At that time, culture was based upon the Byzantine example with traditional practices but opening the way to new ideas. Using the Cyrillic alphabet, the Russian language and literature began to develop, as well.