What were the most important cultural achievements of the Byzantine civilization why?

Byzantium existed for about 1000 years, from 395 to 1453, so it had many achievements:

Code of Laws of Justinian. Advanced for its time, for the 6th century.
Military achievements, for example, Greek fire, which burned enemy ships. It was also used against the Eastern Slavs, for example, against Prince Igor in 941.
Architecture: the walls of Constantinople (were the most powerful at that time in Europe), a domed basilica and cross-domed churches. The most famous building can be considered the Cathedral of St. Sophia.
Mosaic (smalt), icon painting.
Literature: prose, hymns, patericons, hagiography (lives of the saints), chronicles, for example, George Amartola.