Why is justinian important

Justinian I (also known as Justinian the Great) was one of the East Roman/Byzantine Empire's greatest emperors reigning from 527 to 565 AD.Since the Roman Empire split into two, and the West portion has fallen, Justinian saw to it that the Empire should be able to regain its former glory under his rule - thus he was called the "last Roman" by some historians. The emperor was able to reconquer some parts of the fallen West Roman Empire, extending Byzantium's borders as far west as Spain and modern-day Morocco. 

Justinian was also responsible for the uniform rewriting of Roman law (known as the Corpus Juris Civilis), and up to this day, some countries still base their constitution/laws on the Corpus Juris Civilis. During the emperor's reign, Byzantine culture began to emerge, and great churches like the Hagia Sofia were built. However, by 540, the empire was struck by a plague, causing it to plunge into decline.